drm/i915: Parametrize TRANS_DP_PORT_SEL

Parametrize the TRANS_DP_PORT_SEL macros.

v2: WARN for bogus ports (Jani)
    Order the defines mask,value (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180518152931.13104-2-ville.syrjala@linux.intel.com