drm/nv40/fb: use an actual compressed zeta format

SPLIT is apparently just that, and only splits Z and S components.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/subdev/fb/nv40.c

@@ -54,7 +54,7 @@ nv40_fb_tile_comp(struct nouveau_fb *pfb, int i, u32 size, u32 flags,
 	u32 tags  = round_up(tiles / pfb->ram.parts, 0x100);
 	if ( (flags & 2) &&
 	    !nouveau_mm_head(&pfb->tags, 1, tags, tags, 1, &tile->tag)) {
-		tile->zcomp  = 0x24000000; /* Z24S8_SPLIT */
+		tile->zcomp  = 0x28000000; /* Z24S8_SPLIT_GRAD */
 		tile->zcomp |= ((tile->tag->offset           ) >> 8);
 		tile->zcomp |= ((tile->tag->offset + tags - 1) >> 8) << 13;
 #ifdef __BIG_ENDIAN