drm/i915: Replace "cc-option -Wno-foo" with "cc-disable-warning foo"

To quote kbuild/makefiles.txt:

    cc-disable-warning checks if gcc supports a given warning and returns
    the commandline switch to disable it. This special function is needed,
    because gcc 4.4 and later accept any unknown -Wno-* option and only
    warn about it if there is another warning in the source file.

This is exactly what we were trying to achieve with cc-option -Wno-foo and
failed miserably.

Reported-by:  <kbuild-all@01.org>
Fixes: 39bf4de8 ("drm/i915: Add -Wall -Wextra to our build, set warnings to full")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171030172927.18158-1-chris@chris-wilson.co.uk


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>