drm/i915: Handle drm-layer errors in intel_dp_add_mst_connector

Make intel_dp_add_mst_connector handle error returns from the drm_ calls.
Add intel_connector_free to support cleanup on the error path.

v2: Rename new function to avoid confusion, and simplify error
paths (Ville)

v3: Indentation fixup, style fixes (Ville)

v4: Clarify usage of intel_connector_free, and fix usage of
intel_connector_free

v5: Rebase

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171013180144.15865-1-james.ausmus@intel.com