drm/ast: Remove useless drm_fb_get_bpp_depth() call

The function has no side effect and its returned values are ignored,
don't call it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465236852-11710-1-git-send-email-laurent.pinchart@ideasonboard.com