drm/ast: Validate display modes against framebuffer and format limits

Reimplement ast_mode_config_mode_valid() with DRM format helpers and
ast's helpers for framebuffer size calculation. Replaces ast's open-
coded assumptions on bpp and page-alignments.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/r/20250324094520.192974-4-tzimmermann@suse.de