drm/vblank: Fix vblank timestamp debugs

We're currently calling ktime_to_timespec64() on stack garbage
hence the debug output for vblank timestamps also contains garbage.
Let's assing something to the ktime_t first before we go converting
it to a timespec.

While at it micro-optimize the ktime_to_timespec64() calls away
when vblank debugging isn't enabled.

Fixes: 67680d3c0464 ("drm: vblank: use ktime_t instead of timeval")
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Keith Packard <keithp@keithp.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171113150210.11311-1-ville.syrjala@linux.intel.com
Acked-by: Arnd Bergmann <arnd@arndb.de>

drivers/gpu/drm/drm_vblank.c

@@ -664,14 +664,16 @@ bool drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,
 	delta_ns = div_s64(1000000LL * (vpos * mode->crtc_htotal + hpos),
 			   mode->crtc_clock);
 
-	/* save this only for debugging purposes */
-	ts_etime = ktime_to_timespec64(etime);
-	ts_vblank_time = ktime_to_timespec64(*vblank_time);
 	/* Subtract time delta from raw timestamp to get final
 	 * vblank_time timestamp for end of vblank.
 	 */
-	etime = ktime_sub_ns(etime, delta_ns);
-	*vblank_time = etime;
+	*vblank_time = ktime_sub_ns(etime, delta_ns);
+
+	if ((drm_debug & DRM_UT_VBL) == 0)
+		return true;
+
+	ts_etime = ktime_to_timespec64(etime);
+	ts_vblank_time = ktime_to_timespec64(*vblank_time);
 
 	DRM_DEBUG_VBL("crtc %u : v p(%d,%d)@ %lld.%06ld -> %lld.%06ld [e %d us, %d rep]\n",
 		      pipe, hpos, vpos,