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

The driver doesn't need the color depth, only the number of bits per
pixel. Use the right API.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1476744081-24485-8-git-send-email-laurent.pinchart@ideasonboard.com