Do the same as in st_TexSubImage. This fixes
arb_get_texture_sub_image-get on llvmpipe when it is set to prefer
blits, and nouveau when it uses the 3d engine for blits.
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu> Reviewed-by: Marek Olšák <marek.olsak@amd.com>