drm/panic: Add a set_pixel() callback to drm_scanout_buffer

This allows drivers to draw the pixel, and handle tiling, or specific
color formats.

v2:
 * Use fg_color for blit() functions (Javier Martinez Canillas)

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240603095343.39588-3-jfalempe@redhat.com