svga: change enum pipe_resource_usage back to unsigned

This parameter is actually a bitmask of PIPE_TRANSFER_x flags.
Change it back to a simple unsigned type.  IIRC, some compilers
complain about masks of enum values.  Also, this make the function
signature match u_resource_vtbl::transfer_map() again.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/gallium/drivers/svga/svga_resource_buffer.c

@@ -69,7 +69,7 @@ static void *
 svga_buffer_transfer_map(struct pipe_context *pipe,
                          struct pipe_resource *resource,
                          unsigned level,
-                         enum pipe_resource_usage usage,
+                         unsigned usage,
                          const struct pipe_box *box,
                          struct pipe_transfer **ptransfer)
 {