i915: Use struct_mutex to protect ring in GEM mode.

In the conversion for GEM, we had stopped using the hardware lock to protect
ring usage, since it was all internal to the DRM now.  However, some paths
weren't converted to using struct_mutex to prevent multiple threads from
concurrently working on the ring, in particular between the vblank swap handler
and ioctls.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>