drm/i915/bxt: get_hw_state for BXT

Pick appropriate port control register (BXT or VLV), based on device.
Get the current hw state wrt Mipi port.

v2: Rebased on latest drm nightly branch.

v3: Removed the GET_DSI_PORT_CTRL Macro for consistency with earlier
    implementations as per Jani's suggestion.

Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>