gallium/util: make util_copy_framebuffer_state(src=NULL) work

Be more consistent with the other u_inlines util_copy_xyz_state()
helpers and support NULL src.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
1 file changed