Revert "drm/i915/cnl: Use mmio access to context status buffer"

In the previous patch (to include a rmb() after readig the CSB WRITE
pointer from the HWSP) we believe we have fixed the underlying bug, and
so can re-enable using the HWSP on Cannolake.

This reverts commit 61bf9719 ("drm/i915/cnl: Use mmio access to
context status buffer").

References: https://bugs.freedesktop.org/show_bug.cgi?id=105888
References: https://bugs.freedesktop.org/show_bug.cgi?id=106185
References: 61bf9719 ("drm/i915/cnl: Use mmio access to context status buffer")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Timo Aaltonen <tjaalton@ubuntu.com>
Tested-by: Timo Aaltonen <tjaalton@ubuntu.com>
Acked-by: Michel Thierry <michel.thierry@intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180511121147.31915-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -470,9 +470,6 @@ static bool csb_force_mmio(struct drm_i915_private *i915)
 	if (intel_vgpu_active(i915) && !intel_vgpu_has_hwsp_emulation(i915))
 		return true;
 
-	if (IS_CANNONLAKE(i915))
-		return true;
-
 	return false;
 }