The previous calculation (block_stride * block_height * depth) led
to data corruption since we don't take stride into account when
allocating the temporary buffer.
With this fix,
dEQP-GLES3.functional.texture.specification.basic_texsubimage3d.rgba32f_3d
doesn't crash the virtual machine (though the test fails).
Signed-off-by: Dave Airlie <airlied@redhat.com>