drm/i915: add for_each_port_masked macro

Same as for_each_dsi_port, but for general use. Leave the
for_each_dsi_port version around as an "alias" for now to not cause too
much churn. No functional changes.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/a0ef5bf33395e1fcd87178b17d6687b022042cf7.1458070700.git.jani.nikula@intel.com