intel: Request DRI2 buffers for separate stencil and hiz

intel: Request DRI2 buffers for separate stencil and hiz

When it is sensible to do so,
    1) intelCreateBuffer() now attaches separate depth and stencil
       buffers
       to the framebuffer it creates.
    2) intel_update_renderbuffers() requests for the framebuffer
       a separate stencil buffer (DRI2BufferStencil).

The criteria for "sensible" is:
    - The GLX config has nonzero depth and stencil bits.
    - The hardware supports separate stencil.
    - The X driver supports separate stencil, or its support has not yet
      been determined.

If the hardware supports hiz too, then intel_update_renderbuffers()
also requests DRI2BufferHiz.

If after requesting DRI2BufferStencil we determine that X driver did not
actually support separate stencil, we clean up the mistake and never ask
for DRI2BufferStencil again.

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>