st/mesa: fix incorrect glCopyPixels position on fallback path

st/mesa: fix incorrect glCopyPixels position on fallback path

If we hit the pipe_get/put_tile() path for setting up the glCopyPixels
texture we were passing the wrong x/y position to pipe_get_tile().
The x/y position was already accounted for in the pipe_get_transfer()
call so we were effectively reading from 2*readX, 2*readY.

NOTE: This is a candidate for the 7.9 and 7.10 branches.