drm/i915: Unconfuse pre-icl vs. icl+ intel_sagv_{pre,post}_plane_update()

drm/i915: Unconfuse pre-icl vs. icl+ intel_sagv_{pre,post}_plane_update()

intel_sagv_{pre,post}_plane_update() can accidentally forget
to bail out early on pre-icl and proceed down the icl+ codepath
at the end of the function. Fortunately it'll bail out before
it gets too far due to old_qgv_mask==new_qgv_mask==0 so no real
bug here. But lets make the code less confusing anyway.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220214091811.13725-5-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>