drm/i915: New functions added for enabling & disabling MIPI Port Ctrl reg

This patch is in preparation for the DSI dual link
port enable and disable related changes.

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