drm/i915: use upstream version of header tests

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