iris: "Fix" transfer maps of buffers

iris: "Fix" transfer maps of buffers

x should be in bytes, not cpp units

This generally worked out because PIPE_BUFFER is supposedly required
to be R8_UINT or R8_UNORM.  I hear some state trackers pass
PIPE_FORMAT_NONE instead, however, which would make this break.

Just do the right thing directly, to be defensive and clear.