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: 39bf4de89ff7 ("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>

drivers/gpu/drm/i915/Makefile

@@ -12,15 +12,15 @@
 # will most likely get a sudden build breakage... Hopefully we will fix
 # new warnings before CI updates!
 subdir-ccflags-y := -Wall -Wextra
-subdir-ccflags-y += $(call cc-option,-Wno-unused-parameter,)
-subdir-ccflags-y += $(call cc-option,-Wno-type-limits,)
-subdir-ccflags-y += $(call cc-option,-Wno-missing-field-initializers,)
-subdir-ccflags-y += $(call cc-option,-Wno-implicit-fallthrough,)
+subdir-ccflags-y += $(call cc-disable-warning, unused-parameter)
+subdir-ccflags-y += $(call cc-disable-warning, type-limits)
+subdir-ccflags-y += $(call cc-disable-warning, missing-field-initializers)
+subdir-ccflags-y += $(call cc-disable-warning, implicit-fallthrough)
 subdir-ccflags-$(CONFIG_DRM_I915_WERROR) += -Werror
 
 # Fine grained warnings disable
-CFLAGS_i915_pci.o = $(call cc-option,-Wno-override-init,)
-CFLAGS_intel_fbdev.o = $(call cc-option,-Wno-override-init,)
+CFLAGS_i915_pci.o = $(call cc-disable-warning, override-init)
+CFLAGS_intel_fbdev.o = $(call cc-disable-warning, override-init)
 
 subdir-ccflags-y += \
 	$(call as-instr,movntdqa (%eax)$(comma)%xmm0,-DCONFIG_AS_MOVNTDQA)