drm/panic: only draw the foreground color in drm_panic_blit()

The whole framebuffer is cleared, so it's useless to rewrite the
background colored pixels. It allows to simplify the drawing
functions, and prepare the work for the set_pixel() callback.

v2:
 * keep fg16/fg24/fg32 as variable name for the blit function.
 * add drm_panic_is_pixel_fg() to avoid code duplication.
 both suggested by 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/20240603095343.39588-2-jfalempe@redhat.com