drm/i915/guc: Tidy guc_log_control

We plan to decouple log runtime (mapping + relay) from verbosity control.
Let's tidy the code now to reduce the churn in the following patches.

v2: Tidy macros, keep debug messages, use helper var for enable,
    correct typo (Michał)
    Fix incorrect input validaction (Sagar)

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180308154707.21716-1-michal.winiarski@intel.com
diff --git a/drivers/gpu/drm/i915/i915_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index 34d1252..c4cc8fe 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -2499,13 +2499,10 @@ static int i915_guc_log_control_get(void *data, u64 *val)
 {
 	struct drm_i915_private *dev_priv = data;
 
-	if (!HAS_GUC(dev_priv))
+	if (!USES_GUC(dev_priv))
 		return -ENODEV;
 
-	if (!dev_priv->guc.log.vma)
-		return -EINVAL;
-
-	*val = i915_modparams.guc_log_level;
+	*val = intel_guc_log_control_get(&dev_priv->guc);
 
 	return 0;
 }
@@ -2514,10 +2511,10 @@ static int i915_guc_log_control_set(void *data, u64 val)
 {
 	struct drm_i915_private *dev_priv = data;
 
-	if (!HAS_GUC(dev_priv))
+	if (!USES_GUC(dev_priv))
 		return -ENODEV;
 
-	return intel_guc_log_control(&dev_priv->guc, val);
+	return intel_guc_log_control_set(&dev_priv->guc, val);
 }
 
 DEFINE_SIMPLE_ATTRIBUTE(i915_guc_log_control_fops,