drm/panic: Add debugfs entry to test without triggering panic.

Add a debugfs file, so you can test drm_panic without freezing
your machine. This is unsafe, and should be enabled only for
developer or tester.

To display the drm_panic screen on the device 0:
echo 1 > /sys/kernel/debug/dri/0/drm_panic_plane_0

v9:
 * Create a debugfs file for each plane in the device's debugfs
   directory. This allows to test for each plane of each GPU
   independently.

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240409163432.352518-5-jfalempe@redhat.com
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>