ac/surface: Set compressZ for stencil-only surfaces.

We HTILE compress stencil-only surfaces too.

CC: 18.1 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/amd/common/ac_surface.c

@@ -670,7 +670,7 @@ static int gfx6_compute_surface(ADDR_HANDLE addrlib,
 		 config->info.levels == 1);
 
 	AddrSurfInfoIn.flags.noStencil = (surf->flags & RADEON_SURF_SBUFFER) == 0;
-	AddrSurfInfoIn.flags.compressZ = AddrSurfInfoIn.flags.depth;
+	AddrSurfInfoIn.flags.compressZ = !!(surf->flags & RADEON_SURF_Z_OR_SBUFFER);
 
 	/* On CI/VI, the DB uses the same pitch and tile mode (except tilesplit)
 	 * for Z and stencil. This can cause a number of problems which we work