drm/ast: cursor: Move format conversion to shared helper

User-space cursor-image data is encoded in ARBG8888, while hardware
supports ARGB4444. Implement the format conversion as part of the
format-helper framework, so that other drivers can benefit.

This allows to respect the damage area of the cursor update. In
previous code, all cursor image data had to be converted on each
update. Now, only the changed areas require an update. The hardware
image is always updated completely, as it is required for the
checksum update.

The format-conversion helper still contains the old implementation's
optimization of writing 2 output pixels at the same time.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250217122336.230067-3-tzimmermann@suse.de