drm/i915/dsb: Clear DSB_ENABLE_DEWAKE once the DSB is done

drm/i915/dsb: Clear DSB_ENABLE_DEWAKE once the DSB is done

In order to avoid the DSB keeping the DEwake permanently
asserted we must clear DSB_PMCTRL_2.DSB_FORCE_DEWAKE once
we are done. For good measure do the same for
DSB_PMCTRL.DSB_ENABLE_DEWAKE.

Experimentally this doens't seem to be actually necessary
(unlike with DSB_FORCE_DEWAKE). That is, the DSB_ENABLE_DEWAKE
doesn't seem to do anything whenever the DSB is not active.
But I'd hate to waste a ton of power in case there I'm wrong
and there is some way DEwake could remaing asserted. One extra
register write is a small price to pay for some peace of mind.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240624191032.27333-13-ville.syrjala@linux.intel.com
Reviewed-by: Animesh Manna <animesh.manna@intel.com>