drm/stm: Use drm_gem_fb_create()

drm_fb_cma_create() is just a wrapper around drm_gem_fb_create() now,
so use the function directly.

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>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Philippe Cornu <philippe.cornu@st.com>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1502631125-13557-18-git-send-email-noralf@tronnes.org