drm/i915: Force clean compilation with -Werror

Our driver compiles clean (nowadays thanks to 0day) but for me, at least,
it would be beneficial if the compiler threw an error rather than a
warning when it found a piece of suspect code. (I use this to
compile-check patch series and want to break on the first compiler error
in order to fix the patch.)

v2: Kick off a new "Debugging" submenu for i915.ko
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Add "DRM i915" to the menu name as requested by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/Kconfig

@@ -71,3 +71,11 @@ config DRM_I915_PRELIMINARY_HW_SUPPORT
 	  option changes the default for that module option.
 
 	  If in doubt, say "N".
+
+menu "DRM i915 Debugging"
+
+depends on DRM_I915
+
+source drivers/gpu/drm/i915/Kconfig.debug
+
+endmenu

drivers/gpu/drm/i915/Kconfig.debug

+config DRM_I915_WERROR
+	bool "Force GCC to throw an error instead of a warning when compiling"
+	default n
+	---help---
+	  Add -Werror to the build flags for (and only for) i915.ko

drivers/gpu/drm/i915/Makefile

@@ -2,6 +2,8 @@
 # Makefile for the drm device driver.  This driver provides support for the
 # Direct Rendering Infrastructure (DRI) in XFree86 4.1.0 and higher.
 
+subdir-ccflags-$(CONFIG_DRM_I915_WERROR) := -Werror
+
 # Please keep these build lists sorted!
 
 # core driver code