backend-drm: stop direct accessing core struct member

Commit "weston-log: add function to avoid direct
access to compositor members in non-core code" added the
function weston_compositor_add_log_scope mainly to allow
libweston users to avoid direct accessing core structs, as
weston_compositor.

Replace weston_log_context_add_log_scope usage by
weston_compositor_add_log_scope.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>

libweston/backend-drm/drm.c

@@ -2825,8 +2825,7 @@ drm_backend_create(struct weston_compositor *compositor,
 	b->pageflip_timeout = config->pageflip_timeout;
 	b->use_pixman_shadow = config->use_pixman_shadow;
 
-	b->debug = weston_log_ctx_add_log_scope(compositor->weston_log_ctx,
-						"drm-backend",
+	b->debug = weston_compositor_add_log_scope(compositor, "drm-backend",
 						   "Debug messages from DRM/KMS backend\n",
 						   NULL, NULL, NULL);