drm/i915: run kernel-doc on headers as part of HDRTEST

Enabling kernel-doc warnings in commit aaee4bbe ("drm/i915: enable
kernel-doc warnings for CONFIG_DRM_I915_WERROR=y") actually only covers
the .c files. And it's good for avoiding warnings in W= builds. However,
we need something more to check for kernel-doc issues in headers. Add it
as part of the existing HDRTEST.

We have tons of issues, and this unleashes warnings galore on
CONFIG_DRM_I915_WERROR=y. It doesn't fail the build because (at least
for now) we don't pass -Werror to kernel-doc.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230404090528.173075-1-jani.nikula@intel.com

drivers/gpu/drm/i915/Makefile

@@ -387,7 +387,8 @@ always-$(CONFIG_DRM_I915_WERROR) += \
 		$(shell cd $(srctree)/$(src) && find * -name '*.h')))
 
 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 $<; touch $@
+      cmd_hdrtest = $(CC) $(filter-out $(CFLAGS_GCOV), $(c_flags)) -S -o /dev/null -x c /dev/null -include $<; \
+		$(srctree)/scripts/kernel-doc -none $<; touch $@
 
 $(obj)/%.hdrtest: $(src)/%.h FORCE
 	$(call if_changed_dep,hdrtest)