drm/i915/adlp: Assert that VMAs in DPT start at 0

Atm the DPT object can accommodate only one VMA, so the VMA offset will
be always 0. Add an assert for this.

Cc: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210906182715.3915100-4-imre.deak@intel.com

drivers/gpu/drm/i915/display/skl_universal_plane.c

@@ -993,6 +993,11 @@ static u32 skl_surf_address(const struct intel_plane_state *plane_state,
 	u32 offset = plane_state->view.color_plane[color_plane].offset;
 
 	if (intel_fb_uses_dpt(fb)) {
+		/*
+		 * The DPT object contains only one vma, so the VMA's offset
+		 * within the DPT is always 0.
+		 */
+		WARN_ON(plane_state->dpt_vma->node.start);
 		WARN_ON(offset & 0x1fffff);
 		return offset >> 9;
 	} else {