intel: Add assertions to intelCreateBuffer()

intel: Add assertions to intelCreateBuffer()

Assert that the GLX config has an expected depth/stencil bit combination:
one of d24/s8, d16/s0, d0/s0. These are the only depth/stencil
configurations that we advertise.

Remove the check for software stencil, because given the assertions'
constraints the check always fails.

CC: Ian Romanick <idr@freedesktop.org>
CC: Kristian Høgsberg <krh@bitplanet.net>
Acked-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad@chad-versace.us>