drm/fb-cma-helpers: Fix include issue

Exported functions prototypes are missing in drm_fb_cma_helper.c
Include drm_fb_cma_helper to fix that issue.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20191119105753.32363-1-benjamin.gaignard@st.com