drm/i915: Disable -Wuninitialized

This warning is disabled by default in scripts/Makefile.extrawarn when
W= is not provided but this Makefile adds -Wall after this warning is
disabled so it shows up in the build when it shouldn't:

In file included from drivers/gpu/drm/i915/intel_breadcrumbs.c:895:
drivers/gpu/drm/i915/selftests/intel_breadcrumbs.c:350:34: error:
variable 'wq' is uninitialized when used within its own initialization
[-Werror,-Wuninitialized]
        DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
                                        ^~
./include/linux/wait.h:74:63: note: expanded from macro
'DECLARE_WAIT_QUEUE_HEAD_ONSTACK'
        struct wait_queue_head name = __WAIT_QUEUE_HEAD_INIT_ONSTACK(name)
                               ~~~~                                  ^~~~
./include/linux/wait.h:72:33: note: expanded from macro
'__WAIT_QUEUE_HEAD_INIT_ONSTACK'
        ({ init_waitqueue_head(&name); name; })
                                       ^~~~
1 error generated.

Explicitly disable the warning like commit 46e20680 ("drm/i915:
Disable some extra clang warnings").

Link: https://github.com/ClangBuiltLinux/linux/issues/220


Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <nick.desaulniers@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190126071122.24557-1-natechancellor@gmail.com