drm: i915: Use nsec based interfaces

Use ktime_get_raw_ns() and get rid of the back and forth timespec
conversions.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: John Stultz <john.stultz@linaro.org>