drm: sti: Replace drm_fb_get_bpp_depth() with drm_format_plane_cpp()

The driver needs the number of bytes per pixel, not the bpp and depth
info meant for fbdev compatibility. Use the right API.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com>