drm/i915: Add -Wall -Wextra to our build, set warnings to full

Recently W=1 on gcc-7.2 (-Wunused-const-variable) caught a regression
that had been lurking for 6 months, so lets try enabling the full set of
warnings for CI builds. This means more patches will be rejected early
that contain trivial and sometimes not so trivial bugs. However, our
code does not yet compile cleanly with W=1, so we have to apply a filter
to the set of warnings until we can eliminate the mistakes. It also
means that developers will have to be running the full gamut of gcc to
ensure that as warnings come and go with gcc updates, we have the CI
build prepared.

v2: Use fine-grained -Wno overrides. Inside the makefile, we can
specify CFLAGS on a per-object level, which allows us to limit the scope
of any particular warning override.
v3: Place per-file overrides after the main enabling block.
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>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Tomi Sarvela <tomi.p.sarvela@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171024181547.27889-1-chris@chris-wilson.co.ukAcked-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/Makefile

@@ -2,7 +2,26 @@
 # Makefile for the drm device driver.  This driver provides support for the
 # Direct Rendering Infrastructure (DRI) in XFree86 4.1.0 and higher.
 
-subdir-ccflags-$(CONFIG_DRM_I915_WERROR) := -Werror
+# Add a set of useful warning flags and enable -Werror for CI to prevent
+# trivial mistakes from creeping in. We have to do this piecemeal as we reject
+# any patch that isn't warning clean, so turning on -Wall -Wextra (or W=1) we
+# need to filter out dubious warnings.  Still it is our interest
+# to keep running locally with W=1 C=1 until we are completely clean.
+#
+# Note the danger in using -Wall -Wextra is that when CI updates gcc we
+# 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 += $(call cc-option,-Wno-implicit-fallthrough,)
+subdir-ccflags-$(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,)
+
 subdir-ccflags-y += \
 	$(call as-instr,movntdqa (%eax)$(comma)%xmm0,-DCONFIG_AS_MOVNTDQA)