drm/i915: Avoid re-configure panel on every PSR re-enable.

The panel has to be reconfigured only when it really loose the power.
The traditional enable/disable sequence already take care of this so we can
minimize the time spend on every re-enable.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>