i915: Accept GL_DEPTH_STENCIL GL_DEPTH_COMPONENT formats for renderbuffers

i915: Accept GL_DEPTH_STENCIL GL_DEPTH_COMPONENT formats for renderbuffers

Gen2 doesn't support depth/stencil textures, and since

 commit c1d4d4999303f9167b20f4e0674b9436e6295cf7
 Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
 Date:   Thu Apr 24 14:11:43 2014 +0300

    i915: Don't advertise Z formats in TextureFormatSupported on gen2

depth/stencil formats are no longer accepted as texture formats.
However we still want depth/stencil renderbuffers, so add explicit
format checks to intel_alloc_renderbuffer_storage() to allow such
things.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>