drm/i915: Add skl_wm_plane_disable_noatomic()

Add skl_wm_plane_disable_noatomic() which will clear out all
the ddb and wm state for the plane. And let's do this _before_
we call plane->disable_arm() so that it'll actually clear out
the state in the hardware as well.

Currently this won't do anything new for most of the
intel_plane_disable_noatomic() calls since those are done before
wm readout, and thus everything wm/ddb related in the state
will still be zeroed anyway. The only difference will be for
skl_dbuf_sanitize() is happens after wm readout. But I'll be
reordering thigns so that wm readout happens earlier and at that
point this will guarantee that we still clear out the old
wm/ddb junk from the state.

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250306163420.3961-8-ville.syrjala@linux.intel.com