drm: Don't export the drm_fb_get_bpp_depth() function

The function is only used by the drm_helper_mode_fill_fb_struct() core
function to fill the drm_framebuffer bpp and depth fields, used by
drivers that haven't been converted to use pixel formats directly yet.
It should not be used by new drivers, so inline it in its only caller.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1476744081-24485-14-git-send-email-laurent.pinchart@ideasonboard.com