drm/i915: add for_each_port() and use it

Add another for_each style macro to the pile of custom looping macros.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191101134333.11612-1-jani.nikula@intel.com