drm/amdgpu: guard atom_context in devcoredump VBIOS dump

During GPU reset coredump generation, amdgpu_devcoredump_fw_info() unconditionally
dereferences adev->mode_info.atom_context to print VBIOS fields. On reset/teardown
paths this pointer can be NULL, causing a kernel page fault from the deferred
coredump workqueue.

Fix by checking ctx before printing VBIOS fields:

if ctx is valid, print full VBIOS information as before;
This prevents NULL-dereference crashes while preserving coredump output.

Observed page fault log:
[ 667.933329] RIP: 0010:amdgpu_devcoredump_format+0x780/0xc00 [amdgpu]
[ 667.941517] amdgpu 0002:01:00.0: Dumping IP State
[ 667.949660] Code: 8d 57 74 48 c7 c6 01 65 9f c2 48 8d 7d 98 e8 97 96 7a ff 49 8d 97 b4 00 00 00 48 c7 c6 18 65 9f c2 48 8d 7d 98 e8 80 96 7a ff <41> 8b 97 f4 00 00 00 48 c7 c6 2f 65 9f c2 48 8d 7d 98 e8 69 96 7a
[ 667.949666] RSP: 0018:ffffc9002302bd50 EFLAGS: 00010246
[ 667.949673] RAX: 0000000000000000 RBX: ffff888110600000 RCX: 0000000000000000
[ 667.949676] RDX: 000000000000a9b5 RSI: 0000000000000405 RDI: 000000000000a999
[ 667.949680] RBP: ffffc9002302be00 R08: ffffffffc09c3084 R09: ffffffffc09c3085
[ 667.949684] R10: 0000000000000000 R11: 0000000000000004 R12: 00000000000048e0
[ 667.993908] amdgpu 0002:01:00.0: Dumping IP State Completed
[ 667.994229] R13: 0000000000000025 R14: 000000000000000c R15: 0000000000000000
[ 667.994233] FS: 0000000000000000(0000) GS:ffff88c44c2c9000(0000) knlGS:0000000000000000
[ 668.000076] amdgpu 0002:01:00.0: [drm] AMDGPU device coredump file has been created
[ 668.008025] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 668.008030] CR2: 00000000000000f4 CR3: 000000011195f001 CR4: 0000000000770ef0
[ 668.008035] PKRU: 55555554
[ 668.008040] Call Trace:
[ 668.008045] <TASK>
[ 668.016010] amdgpu 0002:01:00.0: [drm] Check your /sys/class/drm/card16/device/devcoredump/data
[ 668.023967] ? srso_alias_return_thunk+0x5/0xfbef5
[ 668.023988] ? __pfx___drm_printfn_coredump+0x10/0x10 [drm]
[ 668.031950] amdgpu 0003:01:00.0: Dumping IP State
[ 668.038159] ? __pfx___drm_puts_coredump+0x10/0x10 [drm]
[ 668.083017] amdgpu 0003:01:00.0: Dumping IP State Completed
[ 668.083824] amdgpu_devcoredump_deferred_work+0x26/0xc0 [amdgpu]
[ 668.086163] amdgpu 0003:01:00.0: [drm] AMDGPU device coredump file has been created
[ 668.095863] process_scheduled_works+0xa6/0x420
[ 668.095880] worker_thread+0x12a/0x270
[ 668.101223] amdgpu 0003:01:00.0: [drm] Check your /sys/class/drm/card24/device/devcoredump/data
[ 668.107441] kthread+0x10d/0x230
[ 668.107451] ? __pfx_worker_thread+0x10/0x10
[ 668.107458] ? __pfx_kthread+0x10/0x10
[ 668.112709] amdgpu 0000:01:00.0: ring vcn_unified_1 timeout, signaled seq=9, emitted seq=10
[ 668.118630] ret_from_fork+0x17c/0x1f0
[ 668.118640] ? __pfx_kthread+0x10/0x10
[ 668.118647] ret_from_fork_asm+0x1a/0x30

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Suggested-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Jesse Zhang <jesse.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>