i965: Only update renderbuffers on initial intelMakeCurrent

i965: Only update renderbuffers on initial intelMakeCurrent

We call intel_prepare_render() in intelMakeCurrent() to make sure we have
renderbuffers before calling _mesa_make_current().  The only reason we
do this is so that we can have valid defaults for width and height.
If we already have buffers for the drawable we're making current, we
don't need this step.

In itself, this is a small optimization, but it also avoids a round trip
that could block on the display server in a unexpected place.

https://bugs.freedesktop.org/show_bug.cgi?id=72540
https://bugs.freedesktop.org/show_bug.cgi?id=72612

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>