gallium: remove pipe_context member from pipe_transfer

gallium: remove pipe_context member from pipe_transfer

There was very little use for this beyond permitting the
pipe_context::tex_transfer_destroy() function to omit the pipe_context
argument.

This change adds the pipe_context argument into tex_transfer_destroy()
so that it looks like other pipe_context functions, and removes the
pipe_context pointer from pipe_transfer.