drm: Add CONFIG_DRM_WERROR

Add kconfig to enable -Werror subsystem wide. This is useful for
development and CI to keep the subsystem warning free, while avoiding
issues outside of the subsystem that kernel wide CONFIG_WERROR=y might
hit.

v2: Don't depend on COMPILE_TEST

Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com> # v1
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/afe5ed943414f7ec3044c1547503b9941686a867.1709629403.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/Kconfig

@@ -415,3 +415,16 @@ config DRM_LIB_RANDOM
 config DRM_PRIVACY_SCREEN
 	bool
 	default n
+
+config DRM_WERROR
+	bool "Compile the drm subsystem with warnings as errors"
+	depends on EXPERT
+	default n
+	help
+	  A kernel build should not cause any compiler warnings, and this
+	  enables the '-Werror' flag to enforce that rule in the drm subsystem.
+
+	  The drm subsystem enables more warnings than the kernel default, so
+	  this config option is disabled by default.
+
+	  If in doubt, say N.

drivers/gpu/drm/Makefile

@@ -30,6 +30,9 @@ subdir-ccflags-y += -Wno-sign-compare
 endif
 # --- end copy-paste
 
+# Enable -Werror in CI and development
+subdir-ccflags-$(CONFIG_DRM_WERROR) += -Werror
+
 drm-y := \
 	drm_aperture.o \
 	drm_atomic.o \