drm/i915: POSTING_READ() in intel_set_memory_cxsr()

We want cxsr exit to happen ASAP, so toss in some POSTING_READ()s to
make sure things are really kicked off.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>