intel: Set num samples for winsys renderbuffers

intel: Set num samples for winsys renderbuffers

Add a new param, num_samples, to intel_create_renderbuffer and
intel_create_private_renderbuffer.

No multisample GL config is yet advertised, so the value of num_samples is
currently 0.  For server-owned winsys buffers, gl_renderbuffer::NumSamples
is not yet used.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com> (v1)
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>