drm/i915: Use frame counter for intel_wait_for_vblank() on CTG

Use the same wait_for_vblank code for CTG that we use for ILK+.

Also fix the name of the frame counter register while at it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>