drm/i915: use kernel-doc -Werror when CONFIG_DRM_I915_WERROR=y

With CONFIG_DRM_I915_WERROR=y, we enable kernel-doc check for both
objects and headers. Now that the kernel-doc warnings have been fixed,
also enable kernel-doc -Werror to fail the build on kernel-doc warnings.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/68ffcad0e6ff2b6cd70c6df28822f967898ce197.1683041799.git.jani.nikula@intel.com

drivers/gpu/drm/i915/Makefile

@@ -374,7 +374,7 @@ obj-$(CONFIG_DRM_I915_GVT_KVMGT) += kvmgt.o
 #
 # Enable locally for CONFIG_DRM_I915_WERROR=y. See also scripts/Makefile.build
 ifdef CONFIG_DRM_I915_WERROR
-    cmd_checkdoc = $(srctree)/scripts/kernel-doc -none $<
+    cmd_checkdoc = $(srctree)/scripts/kernel-doc -none -Werror $<
 endif
 
 # header test
@@ -389,7 +389,7 @@ always-$(CONFIG_DRM_I915_WERROR) += \
 
 quiet_cmd_hdrtest = HDRTEST $(patsubst %.hdrtest,%.h,$@)
       cmd_hdrtest = $(CC) $(filter-out $(CFLAGS_GCOV), $(c_flags)) -S -o /dev/null -x c /dev/null -include $<; \
-		$(srctree)/scripts/kernel-doc -none $<; touch $@
+		$(srctree)/scripts/kernel-doc -none -Werror $<; touch $@
 
 $(obj)/%.hdrtest: $(src)/%.h FORCE
 	$(call if_changed_dep,hdrtest)