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

drivers/gpu/drm/i915/i915_reg.h

@@ -2840,8 +2840,11 @@ enum skl_disp_power_wells {
 
 #define INTERVAL_1_28_US(us)	(((us) * 100) >> 7)
 #define INTERVAL_1_33_US(us)	(((us) * 3)   >> 2)
+#define INTERVAL_0_833_US(us)	(((us) * 6) / 5)
 #define GT_INTERVAL_FROM_US(dev_priv, us) (IS_GEN9(dev_priv) ? \
-				INTERVAL_1_33_US(us) : \
+				(IS_BROXTON(dev_priv) ? \
+				INTERVAL_0_833_US(us) : \
+				INTERVAL_1_33_US(us)) : \
 				INTERVAL_1_28_US(us))
 
 /*