drm/i915/psr: disable aux_frame_sync on psr2 exit

Screen freeze observed if AUX_FRAME_SYNC is not disabled
on psr2 exit.AUX_FRAME_SYNC needed for psr2 is enabled during
psr2 entry. It must be disabled on psr2 exit.

v2: rebase

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jim Bride <jim.bride@linux.intel.com>
Signed-off-by: Vathsala Nagaraju <vathsala.nagaraju@intel.com>
Signed-off-by: Patil Deepti <deepti.patil@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484147673-2044-1-git-send-email-vathsala.nagaraju@intel.com