i965: Fix format of private renderbuffers

i965: 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.

In practice this makes the depth buffer Z24 even if the visual has
depthBits==16.

The incorrect depth buffer format doesn't seem to cause any actual
problems in i965, but it seems like we should fix it anyway. I see
Z16 has been more or less deprecated in the driver except the for
the depthBits==16 case. But if we want to use Z24 even in that
case (not sure it's really legal?) it would look better if the
code made that decision explicitly rather than relying on the
format to get magically overwritten by the renderbuffer code.

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