backend/drm: simplify atomic commit logic

We don't need a per-CRTC atomic request anymore. Let's make the request
per-commit so that it's easier to debug.

This is also groundwork for supporting wlr_output_test properly.

include/backend/drm/drm.h

@@ -57,7 +57,6 @@ struct wlr_drm_crtc {
 	// Atomic modesetting only
 	uint32_t mode_id;
 	uint32_t gamma_lut;
-	drmModeAtomicReq *atomic;
 
 	// Legacy only
 	drmModeCrtc *legacy_crtc;