radeon_pm: fix oops in hwmon_attributes_visible() and radeon_hwmon_show_temp_thresh()

Since commit ec39f64bba34 ("drm/radeon/dpm: Convert to use
devm_hwmon_register_with_groups") radeon_hwmon_init() is using
hwmon_device_register_with_groups(), which sets `rdev' as a device
private driver_data, while hwmon_attributes_visible() and
radeon_hwmon_show_temp_thresh() are still waiting for `drm_device'.

Fix them by using dev_get_drvdata(), in order to avoid this oops:

BUG: unable to handle kernel paging request at 0000000000001e28
IP: [<ffffffffa02ae8b4>] hwmon_attributes_visible+0x18/0x3d [radeon]
PGD 15057e067 PUD 151a8e067 PMD 0
Oops: 0000 [#1] PREEMPT SMP
Call Trace:
internal_create_group+0x114/0x1d9
sysfs_create_group+0xe/0x10
sysfs_create_groups+0x22/0x5f
device_add+0x34f/0x501
device_register+0x15/0x18
hwmon_device_register_with_groups+0xb5/0xed
radeon_hwmon_init+0x56/0x7c [radeon]
radeon_pm_init+0x134/0x7e5 [radeon]
radeon_modeset_init+0x75f/0x8ed [radeon]
radeon_driver_load_kms+0xc6/0x187 [radeon]
drm_dev_register+0xf9/0x1b4 [drm]
drm_get_pci_dev+0x98/0x129 [drm]
radeon_pci_probe+0xa3/0xac [radeon]
pci_device_probe+0x6e/0xcf
driver_probe_device+0x98/0x1c4
__driver_attach+0x5c/0x7e
bus_for_each_dev+0x7b/0x85
driver_attach+0x19/0x1b
bus_add_driver+0x104/0x1ce
driver_register+0x89/0xc5
__pci_register_driver+0x58/0x5b
drm_pci_init+0x86/0xea [drm]
radeon_init+0x97/0x1000 [radeon]
do_one_initcall+0x7f/0x117
load_module+0x1583/0x1bb4
SyS_init_module+0xa0/0xaf

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Alexander Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>