drm/i915: Fix TGL+ plane SAGV watermark programming

drm/i915: Fix TGL+ plane SAGV watermark programming

When we switch between SAGV on vs. off we need to reprogram all
plane wateramrks accordingly. Currently skl_wm_add_affected_planes()
totally ignores the SAGV watermark and just assumes we will use
the normal WM0.

Fix this by utilizing skl_plane_wm_level() which picks the
correct watermark based on use_sagv_wm. Thus we will force
an update on all the planes whose watermark registers need
to be reprogrammed.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210226153204.1270-2-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>