The optimization is available on Ivy Bridge and later, and is disabled by
default.  Enabling it helps certain workloads such as GLBenchmark TRex test.

No piglit regression.

v2
 - no need to save the register before suspend as init_clock_gating can
   correctly program it after resume
 - split IVB change to another commit
Signed-off-by: NChia-I Wu <olv@lunarg.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>