drm/i915: Don't let update_psr function actually enable PSR.

Being more conservative by enabling PSR only on psr_enable function.

Reviewed-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>