drm/ast: cursor: Calculate checksum in helper

Setting the cursor image requires a 32-bit checksum of the cursor
image data. The current cursor code converts the image to ARGB4444
format and computes the checksum in a single step. Moving the
checksum calculation into a separate helper will allow to move the
format conversion into a shared helper.

v2:
- don't loop for checksum'ing final pixel (Jocelyn)
- fix typo in commit message (Jocelyn)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250217122336.230067-2-tzimmermann@suse.de