egl/x11: Implement dri3 support with loader's dri3 helper

egl/x11: Implement dri3 support with loader's dri3 helper

v2: From Martin Peres
 - Tell we are compiling the dri3 backend in configure.ac
 - Update the Makefile.am
 - get rid of the LIBDRM_HAS_RENDERNODE_SUPPORT macro
 - fix some warnings related to EGLuint64KHR to int64_t conversions
 - use dri2_get_dri_config to get the __DRIconfig instead of open-coding it
 - replace the occasional tabs with spaces

v3: From Martin Peres
 - fix and indent problem (Matt Turner)
 - drop the authenticate function, use NULL in the vtable instead (Emil)
 - drop some useless includes (Emil Velikov)
 - mandate libdrm (Emil Velikov)
 - link to xcb-dri3 (Kristian Høgsberg)
 - convert to the new loader interface for drwable (Kristian)
 - remove some dead code after the dropping of some vfuncs (Kristian)
 - add a comment on the topic of rendering to the frontbuffer

v4: From Martin Peres
 - do not expose the preserved swap behavior (Acked by Eric Anholt)

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>