Throw out our local hacks of header tests now that the more generic
kbuild versions are upstream.

At least for now, continue to keep the header tests behind
CONFIG_DRM_I915_WERROR=y knob.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190729140847.18557-1-jani.nikula@intel.com