drm/sti: Remove local fbdev emulation Kconfig option

DRM_STI_FBDEV config is currently used to enable/disable fbdev emulation
for the sti kms driver.

Remove this local config option and use the core fb helpers with
drm_kms_helper.fbdev_emulation module option to enable/disable fbdev
at runtime.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Reviewed-by: Vincent Abriou <vincent.abriou@st.com>