drm/i915: Do both mt and gen6 style forcewake reset on ivb probe

commit 05a2fb15 ("drm/i915: Consolidate forcewake code")
failed to take into account that we have used to reset both
the gen6 style and the multithreaded style forcewake registers.
This is due to fact that ivb can use either, depending on how the
bios has set up the machine.

Mimic the old semantics before we have determined the correct variety
and reset both before the ecobus probe.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Huang Ying <ying.huang@intel.com>
Signed-off-by: NMika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_uncore.c

@@ -1082,8 +1082,14 @@ static void intel_uncore_fw_domains_init(struct drm_device *dev)
 
 		/* We need to init first for ECOBUS access and then
 		 * determine later if we want to reinit, in case of MT access is
-		 * not working
+		 * not working. In this stage we don't know which flavour this
+		 * ivb is, so it is better to reset also the gen6 fw registers
+		 * before the ecobus check.
 		 */
+
+		__raw_i915_write32(dev_priv, FORCEWAKE, 0);
+		__raw_posting_read(dev_priv, ECOBUS);
+
 		fw_domain_init(dev_priv, FW_DOMAIN_ID_RENDER,
 			       FORCEWAKE_MT, FORCEWAKE_MT_ACK);