drm/i915: Macros to convert PM time interval values to microseconds

Added a new GT_PM_INTERVAL_TO_US macro to perform the platform
specific conversion of PM time interval values to microseconds unit.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Akash Goel <akash.goel@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1461350146-23454-1-git-send-email-akash.goel@intel.com


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>