drm/i915/bxt: fix RC6 residency time calculation

The RC6 residency time unit is 833.33ns on BXT according to the
specification, so update the calculation accordingly. Use the same way
as CHV/VLV to divide by the corresponding frequency, as I think this is
the more natural unit for what the HW does internally.

v2:
- add missing IS_BROXTON check (Ville)

Testcase: igt/pm_rc6_residency
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_sysfs.c

@@ -54,6 +54,9 @@ static u32 calc_residency(struct drm_device *dev, const u32 reg)
 
 		if (I915_READ(VLV_COUNTER_CONTROL) & VLV_COUNT_RANGE_HIGH)
 			units <<= 8;
+	} else if (IS_BROXTON(dev)) {
+		units = 1;
+		div = 1200;		/* 833.33ns */
 	}
 
 	raw_time = I915_READ(reg) * units;