drm/stm: Drop drm_gem_prime_export/import

drm/stm: Drop drm_gem_prime_export/import

They're the default.

Aside: Would be really nice to switch the others over to
drm_gem_object_funcs.

Acked-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Yannick Fertre <yannick.fertre@st.com>
Cc: Philippe Cornu <philippe.cornu@st.com>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Vincent Abriou <vincent.abriou@st.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: linux-stm32@st-md-mailman.stormreply.com
Cc: linux-arm-kernel@lists.infradead.org
Link: https://patchwork.freedesktop.org/patch/msgid/20190614203615.12639-31-daniel.vetter@ffwll.ch