drm: Use ptrdiff_t instead of intmax_t in format string

since we're looking at pointer differences.

Otherwise the build fails on arm like

In file included from ../backend/drm/drm.c:19:0:
../include/wlr/util/log.h:34:17: error: format '%jd' expects argument of type 'intmax_t', but argument 7 has type 'int' [-Werror=format=]
  _wlr_log(verb, "[%s:%d] " fmt, _strip_path(__FILE__), __LINE__, ##__VA_ARGS__)
                 ^
../backend/drm/drm.c:462:2: note: in expansion of macro 'wlr_log'
  wlr_log(L_DEBUG, "%s: crtc=%ju ovr=%jd pri=%jd cur=%jd", conn->output.name,
  ^~~~~~~
../backend/drm/drm.c:462:39: note: format string is defined here
  wlr_log(L_DEBUG, "%s: crtc=%ju ovr=%jd pri=%jd cur=%jd", conn->output.name,
                                     ~~^
                                     %d

backend/drm/drm.c

@@ -459,7 +459,7 @@ static bool wlr_drm_connector_set_mode(struct wlr_output *output,
 	if (!crtc) {
 		return false;
 	}
-	wlr_log(L_DEBUG, "%s: crtc=%ju ovr=%jd pri=%jd cur=%jd", conn->output.name,
+	wlr_log(L_DEBUG, "%s: crtc=%td ovr=%td pri=%td cur=%td", conn->output.name,
 		crtc - drm->crtcs,
 		crtc->overlay ? crtc->overlay - drm->overlay_planes : -1,
 		crtc->primary ? crtc->primary - drm->primary_planes : -1,