drm/i915: Rewrite mst suspend/resume in terms of encoders

Rather than looping over all the ports and picking the encoder based on
the port, let's just loop over all the encoders instead. Gets rid of
some irq_port[] usage, which is a bit of an eye sore.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180705164357.28512-4-ville.syrjala@linux.intel.com
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>