drm/vkms: isolate pixel conversion functionality

Currently, the pixel conversion functions repeat the same loop to
iterate the rows. Instead of repeating the same code for each pixel
format, create a function to wrap the loop and isolate the pixel
conversion functionality.

Suggested-by: Arthur Grillo <arthurgrillo@riseup.net>
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/20230418130525.128733-2-mcanal@igalia.com