drm/vblank: Fix vblank timestamp debugs

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: 6c312ab45b46 ("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>