drm/ast: Handle framebuffer from dma-buf

drm/ast: Handle framebuffer from dma-buf

In the atomic update callback, ast should call
drm_gem_fb_begin_cpu_access() to make sure it can read the
framebuffer from the CPU, otherwise the data might not be there due
to cache, and synchronization.

Tested on a Lenovo SE100, while rendering on the ArrowLake GPU with
i915 driver, and using ast for display.

Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20251030091627.340780-1-jfalempe@redhat.com
Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>