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.