drm/i915: Enable MIPI PHY transparent latch for DSI Port C

Common bit to be used for both DSI Port A & DSI Port C.

Signed-off-by: Gaurav K Singh <gaurav.k.singh@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>