Revert "backend/drm: Automatic non-blocking commits"

This reverts commit 45ba35719e.

Sadly, this causes regressions on amdgpu [1] and even with these
fixed, there are fundamental issues with non-blocking modesets [2].

[1]: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3745
[2]: https://oftc.irclog.whitequark.org/dri-devel/2023-10-11#1697031838-1697036920;

backend/drm/atomic.c

@@ -68,21 +68,6 @@ static bool atomic_commit(struct atomic *atom,
 	}
 
 	int ret = drmModeAtomicCommit(drm->fd, atom->req, flags, drm);
-
-	// A non-blocking commit will fail with EBUSY if a commit is already in
-	// progress for the affected CRTCs, and the caller is responsible for
-	// waiting for page_flip events before queuing more non-blocking commits.
-	//
-	// The driver may implicitly add more CRTCs to our commit for e.g. resource
-	// reallocation, and in that case we have no chance of waiting for the
-	// right page_flip events. Retry with a blocking commit when this happens.
-	if (ret == -EBUSY &&
-			(flags & DRM_MODE_ATOMIC_NONBLOCK) &&
-			(flags & DRM_MODE_ATOMIC_ALLOW_MODESET)) {
-		flags &= ~DRM_MODE_ATOMIC_NONBLOCK;
-		ret = drmModeAtomicCommit(drm->fd, atom->req, flags, drm);
-	}
-
 	if (ret != 0) {
 		wlr_drm_conn_log_errno(conn,
 			(flags & DRM_MODE_ATOMIC_TEST_ONLY) ? WLR_DEBUG : WLR_ERROR,
@@ -329,8 +314,12 @@ static bool atomic_crtc_commit(struct wlr_drm_connector *conn,
 	}
 	if (modeset) {
 		flags |= DRM_MODE_ATOMIC_ALLOW_MODESET;
-	}
+	} else if (!test_only && (state->base->committed & WLR_OUTPUT_STATE_BUFFER)) {
-	if (!test_only && conn->pending_page_flip_crtc == 0) {
+		// The wlr_output API requires non-modeset commits with a new buffer to
+		// wait for the frame event. However compositors often perform
+		// non-modesets commits without a new buffer without waiting for the
+		// frame event. In that case we need to make the KMS commit blocking,
+		// otherwise the kernel will error out with EBUSY.
 		flags |= DRM_MODE_ATOMIC_NONBLOCK;
 	}