drm/i915: Squelch very verbose error logging

Having found the error causing the IGT test to fail, downgrade the
verbose logging so that we stop flooding the syslogs as we deliberately
provoke it many thousands of time during selftests.

References: 10195b1e44 ("drm/i915: Show vma allocator stack when in doubt")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180706065332.15214-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/Kconfig.debug

@@ -51,6 +51,18 @@ config DRM_I915_DEBUG_GEM
 
           If in doubt, say "N".
 
+config DRM_I915_ERRLOG_GEM
+	bool "Insert extra logging (very verbose) for common GEM errors"
+	default n
+	depends on DRM_I915_DEBUG_GEM
+	help
+	  Enable additional logging that may help track down the cause of
+	  principally userspace errors.
+
+	  Recommended for driver developers only.
+
+	  If in doubt, say "N".
+
 config DRM_I915_TRACE_GEM
 	bool "Insert extra ftrace output from the GEM internals"
 	depends on DRM_I915_DEBUG_GEM