drm/vkms: Use const for input pointers in pixel_read an pixel_write functions

As the pixel_read and pixel_write function should never modify the input
buffer, mark those pointers const.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241118-yuv-v14-4-2dbc2f1e222c@bootlin.com


Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>