u_blitter: port radv 3D blit coords logic.

The current code fails a lot of VK CTS tests, this fixes them all:
dEQP-VK*blit_image*3d*

Cc: 20.3 <mesa-stable>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7499>