drm/i915: Only grab timestamps when needed

No need to call ktime_get_raw_ns twice per unlimited wait and can
also elimate a local variable.

v2: Added comment about silencing the compiler warning. (Daniel Vetter)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452870672-13901-1-git-send-email-tvrtko.ursulin@linux.intel.com