drm/vkms: Isolate writeback pixel conversion functions

All convertions from the ARGB16161616 format follow the same structure.
Instead of repeting the same structure for each supported format, create
a function to encapsulate the common logic and isolate the pixel
conversion functions in a callback function.

Suggested-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Arthur Grillo <arthurgrillo@riseup.net>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230515135204.115393-4-mcanal@igalia.com