drm/i915: clean up power sequencing register port select definitions

Remove duplicates, add VLV specific macros for port B and C.

v2: also add PANEL_PORT_SELECT_DPC_VLV for clarity (Ville)

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>