drm/mgag200: Add drm_panic support

drm/mgag200: Add drm_panic support

Add support for the drm_panic module, which displays a message to
the screen when a kernel panic occurs.

v5:
 * Also check that the plane is visible and primary. (Thomas Zimmermann)

v7:
 * use drm_for_each_primary_visible_plane()

v8:
 * Replace get_scanout_buffer() logic with drm_panic_set_buffer()
   (Thomas Zimmermann)

v9:
 * Revert to using get_scanout_buffer() (Sima)
 * move get_scanout_buffer() to plane helper functions (Thomas Zimmermann)

v12:
 * Use array for map and pitch in struct drm_scanout_buffer
   to support multi-planar format later. (Thomas Zimmermann)

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240409163432.352518-8-jfalempe@redhat.com
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>