drm: Move EXPORT_SYMBOL_FOR_TESTS_ONLY under a separate Kconfig

Currently, we only export symbols for drm-selftests which are either
compiled as modules or into the main drm builtin. However, if we want to
export symbols from drm.ko for the drivers' selftests, we require a
means of controlling that export separately. So we add a new Kconfig to
determine whether or not the EXPORT_SYMBOL_FOR_TESTS_ONLY() takes
effect.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107180601.30815-1-chris@chris-wilson.co.uk