i965: Instead of fallback on missing region, just bind a null renderbuffer.

i965: Instead of fallback on missing region, just bind a null renderbuffer.

The change for GPU hanging in 13bab58f04c1ec6d0d52760eab490a0997d9abe2
fell back even when rb == NULL, which is wrong for GLES2 and caused
segfaulting in GLES2 conformance.  For the GPU hang case (where the
broken 2D driver failed to allocate a BO for the window system
renderbuffer), it also would assertion fail/segfault immediately after
the fallback setup when the renderbuffer map failed.

Fixes GLES2 conformance packed_depth_stencil.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>