iris: Default to X-tiling for scanout buffers without modifiers

Neither Mutter nor KWin's wayland compositors appear to use modifiers.
In the non-modifier case, iris was still trying to use Y-tiling for
scan-out surfaces, leading to this error:

(gnome-shell:7247): mutter-WARNING **: 09:23:47.787: meta_drm_buffer_gbm_new failed: drmModeAddFB failed: Invalid argument

We now fall back to the historical X-tiling for scanout buffers, which
ought to work everyone, at lower performance.  To regain that, we need
to ensure modifiers are actually supported in environments people use.

Fixes: fbf3124771 ("iris: Rework tiling/modifiers handling")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/gallium/drivers/iris/iris_resource.c

@@ -807,6 +807,8 @@ iris_resource_create_with_modifiers(struct pipe_screen *pscreen,
       if (templ->usage == PIPE_USAGE_STAGING ||
           templ->bind & (PIPE_BIND_LINEAR | PIPE_BIND_CURSOR) )
          tiling_flags = ISL_TILING_LINEAR_BIT;
+      else if (templ->bind & PIPE_BIND_SCANOUT)
+         tiling_flags = ISL_TILING_X_BIT;
    }
 
    isl_surf_usage_flags_t usage = pipe_bind_to_isl_usage(templ->bind);