dri3: Return the current swap interval from glXGetSwapIntervalMESA().

We were caching only the value set with glXSwapIntervalSGI(), missing out
on the default setting of the swap interval by the loader.  This fixes
glxgears's warning about being vblank synchronized by default.

Fixes: 9777c423 ("loader: drop the [gs]et_swap_interval callbacks")
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit edc7deec)