drm/i915/mst: abstract intel_dp_mst_source_support()

Add a function for checking source MST support. Drop intel_dp->can_mst
and use intel_dp->mst_mgr.cbs to indicate the same. It's the single
point of truth without additional state variables. In code, "source
support" is also self-documenting as opposed to the vague "can mst".

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/20211006101618.22066-1-jani.nikula@intel.com