In the call arguments to dri2_create_drawable decouple loaderPrivate
from dri2_surf. For all callers of dri2_create_drawable the two
pointers are the same with the exception of the gbm backed platform.
Let the calling code of dri2_create_drawable decide what
loaderPrivate shall be.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>