drm/msm/dpu: Propagate error from dpu_assign_plane_resources

The dpu_plane_virtual_assign_resources function might fail if there is
no suitable SSPP(s) for the plane. This leaves sspp field in plane
state uninitialized and later leads to NULL dereference during commit:

Call trace:
_dpu_crtc_blend_setup+0x194/0x620 [msm] (P)
dpu_crtc_atomic_begin+0xe4/0x240 [msm]
drm_atomic_helper_commit_planes+0x88/0x358
msm_atomic_commit_tail+0x1b4/0x8b8 [msm]
commit_tail+0xa8/0x1b0
drm_atomic_helper_commit+0x180/0x1a0
drm_atomic_commit+0x94/0xe0
drm_mode_atomic_ioctl+0xa88/0xd60
drm_ioctl_kernel+0xc4/0x138
drm_ioctl+0x364/0x4f0
__arm64_sys_ioctl+0xac/0x108
invoke_syscall.constprop.0+0x48/0x100
el0_svc_common.constprop.0+0x40/0xe8
do_el0_svc+0x24/0x38
el0_svc+0x30/0xe0
el0t_64_sync_handler+0xa0/0xe8
el0t_64_sync+0x198/0x1a0

Fixes: 3ed12a3664b3 ("drm/msm/dpu: allow sharing SSPP between planes")
Signed-off-by: Vladimir Lypak <vladimir.lypak@gmail.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/681916/
Link: https://lore.kernel.org/r/20251017-b4-dpu-fixes-v1-2-40ce5993eeb6@gmail.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>