drm: unbreak the DRM menu, broken by DRM_EXPORT_FOR_TESTS

Unbreak the DRM menu. This Kconfig symbol does not depend on DRM,
so the menu is broken at that point.

Move the symbol to a location in the Kconfig file so that it does
not break the dependency continuity.

Fixes: 6349120d ("drm: Move EXPORT_SYMBOL_FOR_TESTS_ONLY under a separate Kconfig")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/04221997-79ba-f8a2-4f2d-3c3d9f5219bc@infradead.org