i915: Fix format of private renderbuffers

i915: Fix format of private renderbuffers

intel_alloc_renderbuffer_storage() will clobber rb->Format which was
already set up by intel_create_renderbuffer(). This causes the driver
to potentially create the depth buffer in the wrong format.

Long time ago things worked by accident because
_mesa_choose_tex_format() checked for ARB_depth_texture
and thus returned MESA_FORMAT_NONE on gen2 hardware. Somehow
that ended up working when depthBits==16 because the driver
would then pick DEPTH_FRMT_16_FIXED. Not sure how, but things
also seemed to work with depthBits==24.

Things started to go more sideways at:
 commit 6ae473221a53d8bcb584021483c5328797c6b67c
 Author: Eric Anholt <eric@anholt.net>
 Date:   Mon Apr 22 16:04:25 2013 -0700

    intel: Fold the one last function intel_tex_format.c into the caller.

since that caused intel_miptree_create_layout() to divide by zero
when encoutering MESA_FORMAT_NONE (bw==0). So after this
commit things were broken enough that many applications wouldn't even
run.

Things got a bit better at:
 commit c245efe7e8247ba0c845dee7b77e63fdbfc7e1b3
 Author: Eric Anholt <eric@anholt.net>
 Date:   Thu Mar 21 09:50:45 2013 -0700

    mesa: Remove extension checking from ChooseTexFormat.

since now _mesa_choose_tex_format() would return MESA_FORMAT_X8_Z24
for GL_DEPTH_COMPONENT due to i915 erroneosly claiming that
MESA_FORMAT_X8_S24 (and others) are supported texture formats even
on gen2 hardware. So now the the div-by-zero was gone, but now the
driver would pick DEPTH_FRMT_24_FIXED_8_OTHER even when
depthBits==16 which caused rendering problems.

If we prevent rb->Format from getting clobbered for the depth buffer
things work much better. This makes the spinning title text visible
again in chromium-bsu at 16bpp, for example.

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