drm/i915: Limit number of reads to stabilize rc6 counter reads

We have only 8bits of precise timestamps in which to complete our
upper/load reads, along with the switch between precision. This is not
always enough time to read the upper counter twice within the same time
slice, leading to hard lockups. Limit the number of times to prevent
an inifite loop (my fault for assuming we would have no trouble doing
the write + reads fast enough).

Fixes: 47c21d9a1a7b ("drm/i915: Extend vlv/chv residency resolution")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100377
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170324165418.7455-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

drivers/gpu/drm/i915/intel_pm.c

@@ -8388,6 +8388,7 @@ static u64 vlv_residency_raw(struct drm_i915_private *dev_priv,
 			     const i915_reg_t reg)
 {
 	u32 lower, upper, tmp;
+	int loop = 2;
 
 	/* The register accessed do not need forcewake. We borrow
 	 * uncore lock to prevent concurrent access to range reg.
@@ -8416,7 +8417,7 @@ static u64 vlv_residency_raw(struct drm_i915_private *dev_priv,
 		I915_WRITE_FW(VLV_COUNTER_CONTROL,
 			      _MASKED_BIT_ENABLE(VLV_COUNT_RANGE_HIGH));
 		upper = I915_READ_FW(reg);
-	} while (upper != tmp);
+	} while (upper != tmp && --loop);
 
 	/* Everywhere else we always use VLV_COUNTER_CONTROL with the
 	 * VLV_COUNT_RANGE_HIGH bit set - so it is safe to leave it set