drm/i915/dp: Power down sink before disable pipe/transcoder clock

Disabling pipe/transcoder clock before power down sink could cause
sink lost signal, causing it to trigger a hotplug to notify source
that link signal was lost.

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/20191202222513.337777-2-jose.souza@intel.com