winsys/svga: use new pb_usage_flags enum type

winsys/svga: use new pb_usage_flags enum type

And add a comment that we're implicitly converting PIPE_TRANSFER_
flags to PB_USAGE_ flags in one place.  And statically assert that
the enum values match.

Reviewed-by: Neha Bhende <bhenden@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>