Staging: udlfb: pre-allocated urb list helpers

Add functions to pre-allocate and free usb bulk urbs for core render path.

Udlfb currently allocates a single urb, guarded by a mutex, that is a key
bottleneck. Because udlfb sends so much data, preallocation is most efficient.

Functions will be used by new rendering functions in later patches.

Signed-off-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>