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>