glx/dri3: Convert to use dri3 helper in loader library

glx/dri3: Convert to use dri3 helper in loader library

v2: From Martin Peres
 - convert to the new drawable interface
 - delete dead code after the dropping of some vfuncs
 - delete the width and height attributes since they are found in the helper

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>