egl_dri2: Add a function to let platform code return dri drawable from _EGLSurface

egl_dri2: Add a function to let platform code return dri drawable from _EGLSurface

dri3 for EGL will use different struct other than dri2_egl_surface for
an EGL surface, the common code only uses __DRIdrawable from that
struct, so instead of converting _EGLSurface to dri2_egl_surface, let
the platform code return the __DRIdrawable by its own (although the
current platforms use the same function).

v2: From Martin Peres
 - convert to the new drawable interface (Kristian)

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>