drm: xilinx: drv: Make some format functions external

xilinx_drm_format_bpp() and xilinx_drm_format_depth() can be referred
externally, when framebuffer is allocated and configured outside.
Make these functions external.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>