loader: Add dri3 helper

loader: Add dri3 helper

v2: From Martin Peres
 - Try to fit in the 80-col limit as much as possible

v3: From Martin Peres
 - introduce loader_dri3_helper.la to avoid dragging the xcb dep everywhere (Kristian & Emil)
 - get rid of the width, height, dri_screen and is_different_gpu vfuncs (Kristian)
 - replace the create/destroy functions with init/fini for dri3 drawables
 - prefix static functions with dri3_ and exported ones with loader_dri3 (Emil)
 - keep the function definition consistent (Emil)

Signed-off-by: Boyan Ding <boyan.j.ding@gmail.com>
Signed-off-by: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.co.uk>