drm: move EXPORT_SYMBOL_FOR_TESTS_ONLY to drm_util.h

In the quest to get rid of drmP.h move the newly
added EXPORT_SYMBOL_FOR_TESTS_ONLY to drm_util.h.
Fix the single user.

Add a note to drmP.h to avoid further use of it.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190112193251.20450-3-sam@ravnborg.org