drm/i915: Add wait_for_us

This is for callers who want micro-second precision but are not
waiting from the atomic context.

v2:
  * Fix atomic waits. (Dave Gordon)
  * Use USEC_PER_SEC and USEC_PER_MSEC. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>