drm/i915: add update function to disable/enable-back PSR

drm/i915: add update function to disable/enable-back PSR

Required function to disable PSR when going to console mode.
But also can be used whenever PSR mode entry conditions changed.

v2: Add it before PSR Hook. Update function not really been called yet.
v3: Fix coding style detected by checkpatch by Paulo Zanoni.
v4: do_enable must be static as Paulo noticed.

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