drm/i915: Disable some extra clang warnings

Commit 39bf4de8

 ("drm/i915: Add -Wall -Wextra to our build, set
warnings to full") enabled extra warnings for i915 to spot possible
bugs in new code, and then disabled a subset of these warnings to keep
the current code building without warnings (with gcc). Enabling the
extra warnings also enabled some additional clang-only warnings, as a
result building i915 with clang currently is extremely noisy. For now
also disable the clang warnings sign-compare, sometimes-uninitialized,
unneeded-internal-declaration and initializer-overrides. If desired
they can be re-enabled after the code has been fixed.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180501182440.70121-1-mka@chromium.org