virgl: Only PIPE_BUFFER with VIRGL_BIND_CUSTOM flag is considered busy during creation

virgl: Only PIPE_BUFFER with VIRGL_BIND_CUSTOM flag is considered busy during creation

When the virglrenderer performs the attach action for the PIPE_BUFFER
with the VIRGL_BIND_CUSTOM flag, it will synchronize the data from
res->ptr to res->iov, at this time we need to treat it as busy,
otherwise it will cause some race conditions.

This optimizes commit:
fe9333f7b5e ("virgl: Set res->maybe_busy to true when creating resources")

Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18098>