drm: sti: remove sti_gem_prime_export hack

Thanks to "drm: prime: Honour O_RDWR during prime-handle-to-fd"
commit we don't need to hack flags anymore.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Vincent Abriou <vincent.abriou@st.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>