drm/i915/guc: Don't read SOFT_SCRATCH(15) on MMIO error

SOFT_SCRATCH(15) is used by GuC for sending MMIO GuC events to host and
those events are now handled by intel_guc_to_host_event_handler_mmio().

We should not try to read it on MMIO action error as 1) we may be using
different set of registers for GuC MMIO communication, and 2) GuC may
use CTB mechanism for sending events to host.

While here, upgrade error message to DRM_ERROR.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180528171618.10436-1-michal.wajdeczko@intel.com

drivers/gpu/drm/i915/intel_guc.c

@@ -346,10 +346,8 @@ int intel_guc_send_mmio(struct intel_guc *guc, const u32 *action, u32 len,
 		ret = -EIO;
 
 	if (ret) {
-		DRM_DEBUG_DRIVER("INTEL_GUC_SEND: Action 0x%X failed;"
-				 " ret=%d status=0x%08X response=0x%08X\n",
-				 action[0], ret, status,
-				 I915_READ(SOFT_SCRATCH(15)));
+		DRM_ERROR("MMIO: GuC action %#x failed with error %d %#x\n",
+			  action[0], ret, status);
 		goto out;
 	}