drm/panic: Add a kmsg panic screen

Add a kmsg option, which will display the last lines of kmsg,
and should be similar to fbcon.
Add a drm.panic_screen module parameter, so you can choose between
the different panic screens available.
two options currently, but more will be added later:
 * "user": a short message telling the user to reboot the machine.
 * "kmsg": fill the screen with the last lines of kmsg.

You can even change it at runtime by writing to
/sys/module/drm/parameters/panic_screen

v2:
 * Use module parameter instead of Kconfig choice
   (Javier Martinez Canillas)

v3:
 * Add help for module parameter usage in Kconfig
   (Javier Martivez Canillas)
 * Fix use of font->height before checking it isn't null.
   (kernel test robot, Dan Carpenter)

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240603095343.39588-4-jfalempe@redhat.com