drm/i915/display/mst: Move DPMS_OFF call to post_disable

Moving just to simplify handling as there is no change in behavior.

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191202222513.337777-3-jose.souza@intel.com