mesa: Fix ReadBuffers with pbuffers

pbuffers are internally single-buffered.  Marek fixed DrawBuffers to
handle this case, but we need to fix ReadBuffers too.  Otherwise,
pretty much every conformance test fails because glReadPixels breaks.

v2: Refactor the switch into a helper (suggested by Eric Anholt)

Fixes: 35294f2e ("mesa: fix pbuffers because internally they are front buffers")
Acked-by: Eric Engestrom <eric.engestrom@intel.com> (v1)
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 78164a3a)