drm-backend: require DRM_CAP_TIMESTAMP_MONOTONIC

Weston internals and Wayland clients assume that output presentation
clock cannot go backwards. Therefore require unconditionally that KMS
uses the monotonic clock.

The kernel unconditionally supports DRM_CAP_TIMESTAMP_MONOTONIC. See:

commit 25e1a79874eb3821d93310c908cc0a81b47af060
Author: Arnd Bergmann <arnd@arndb.de>
Date:   Wed Oct 11 17:20:13 2017 +0200

    drm: vblank: remove drm_timestamp_monotonic parameter

That one removed the final possibility of DRM_CAP_TIMESTAMP_MONOTONIC
being false, by removing the module option. But even before that, all
drivers have been supporting monotonic, since

commit c61eef726a78ae77b6ce223d01ea2130f465fe5c
Author: Imre Deak <imre.deak@intel.com>
Date:   Tue Oct 23 18:53:26 2012 +0000

    drm: add support for monotonic vblank timestamps

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

libweston/backend-drm/kms.c

@@ -1421,19 +1421,17 @@ init_kms_caps(struct drm_backend *b)
 {
 	uint64_t cap;
 	int ret;
-	clockid_t clk_id;
 
 	weston_log("using %s\n", b->drm.filename);
 
 	ret = drmGetCap(b->drm.fd, DRM_CAP_TIMESTAMP_MONOTONIC, &cap);
-	if (ret == 0 && cap == 1)
-		clk_id = CLOCK_MONOTONIC;
-	else
-		clk_id = CLOCK_REALTIME;
+	if (ret != 0 || cap != 1) {
+		weston_log("Error: kernel DRM KMS does not support DRM_CAP_TIMESTAMP_MONOTONIC.\n");
+		return -1;
+	}
 
-	if (weston_compositor_set_presentation_clock(b->compositor, clk_id) < 0) {
-		weston_log("Error: failed to set presentation clock %d.\n",
-			   clk_id);
+	if (weston_compositor_set_presentation_clock(b->compositor, CLOCK_MONOTONIC) < 0) {
+		weston_log("Error: failed to set presentation clock to CLOCK_MONOTONIC.\n");
 		return -1;
 	}