drm/i915: Provide more clues as to why MST is/is not used

Always print out the information whether the port and sink can each
do MST. And let's include the modparam in the debug output as well.
Makes life a little less confusing when you don't have to wonder
why MST isn't kicking in.

This does cause a slight change in our behaviour towards the sink.
Previously we only read the MSTM_CAP register after passing all
the other checks. Now we will read that register regardless. Hopefully
some crazy sink doesn't get confused by a simple register read.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181003184210.1306-1-ville.syrjala@linux.intel.com
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>