drm/i915: Add GEM debugging Kconfig option

Currently there is a #define to enable extra BUG_ON for debugging
requests and associated activities. I want to expand its use to cover
all of GEM internals (so that we can saturate the code with asserts).
We can add a Kconfig option to make it easier to enable - with the usual
caveats of not enabling unless explicitly requested.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460565315-7748-3-git-send-email-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/Kconfig.debug b/drivers/gpu/drm/i915/Kconfig.debug
index 61485c8..8f40410 100644
--- a/drivers/gpu/drm/i915/Kconfig.debug
+++ b/drivers/gpu/drm/i915/Kconfig.debug
@@ -27,3 +27,15 @@
 
           If in doubt, say "N".
 
+config DRM_I915_DEBUG_GEM
+        bool "Insert extra checks into the GEM internals"
+        default n
+        depends on DRM_I915_WERROR
+        help
+          Enable extra sanity checks (including BUGs) along the GEM driver
+          paths that may slow the system down and if hit hang the machine.
+
+          Recommended for driver developers only.
+
+          If in doubt, say "N".
+