gallium: unify transfer functions

gallium: unify transfer functions

"get_transfer + transfer_map" becomes "transfer_map".
"transfer_unmap + transfer_destroy" becomes "transfer_unmap".

transfer_map must create and return the transfer object and transfer_unmap
must destroy it.

transfer_map is successful if the returned buffer pointer is not NULL.
If transfer_map fails, the pointer to the transfer object remains unchanged
(i.e. doesn't have to be NULL).

Acked-by: Brian Paul <brianp@vmware.com>