drm/i915/bxt: Set time interval unit to 0.833us

Note that in Bspec you have to dig around in a section called
"Timestamp bases" and Bspec update request is filed.

Signed-off-by: Ankitprasad Sharma <ankitprasad.r.sharma@intel.com>
Signed-off-by: Akash Goel <akash.goel@intel.com>
Signed-off-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
[danvet: Add note about state of Bspec.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>