drm/i915: Also disable PSR on Sink when disabling it on Source.

It is not a bad idea to disable the PSR feature on Sink
when we are disabling on the Source.

v2: Move dpcd write inside mutex protected area as suggested by Sonika.

Cc: Sonika Jindal <sonika.jindal@intel.com>
Suggested-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>