drm/i915/mst: abstract handling of link status in DP MST

We'll want to expand on this, so abstract it to a separate function
first. Improve debug logging while at it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220112110319.1172110-4-jani.nikula@intel.com