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

At this point, we applied it to the kernel and promptly kicked it out
again as it broke buildbots (due to a compiler warning on 32bits):

commit 908d759b
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue May 26 07:46:21 2015 +0200

    Revert "drm/i915: Force clean compilation with -Werror"

v3: Avoid enabling -Werror for allyesconfig/allmodconfig builds, using
COMPILE_TEST as a suitable proxy suggested by Andrew Morton. (Damien)
Only make the option available for EXPERT to reinforce that the option
should not be casually enabled.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1460565315-7748-1-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/Kconfig.debug

+config DRM_I915_WERROR
+        bool "Force GCC to throw an error instead of a warning when compiling"
+        # As this may inadvertently break the build, only allow the user
+        # to shoot oneself in the foot iff they aim really hard
+        depends on EXPERT
+        # We use the dependency on !COMPILE_TEST to not be enabled in
+        # allmodconfig or allyesconfig configurations
+        depends on !COMPILE_TEST
+        default n
+        help
+          Add -Werror to the build flags for (and only for) i915.ko.
+          Do not enable this unless you are writing code for the i915.ko module.
+
+          Recommended for driver developers only.
+
+          If in doubt, say "N".
+
 config DRM_I915_DEBUG
         bool "Enable additional driver debugging"
         depends on DRM_I915

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