drm/vkms: Add documentation

Add documentation around vkms_output and its initialization.
Add some documentation on pixel conversion functions.
Update of outdated comments for pixel_write functions.

Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240905-google-clarifications-v4-1-e43c1f2e3d87@bootlin.com