ir3/compiler: Use the NIR computed offset for image store/atomics

Remove the offset computation from the a4xx/a5xx backend and use the
one pre-computed in NIR by lower_io_offsets pass.

No regressions observed on affected tests from Khronos CTS and piglit
suites, compared to master.

Unfortunately shader-db is not helpful for stats in this case. Few
shaders there exercise image store or image atomic, and of those that
do, most require higher versions of GLSL than 3.10, so they get skipped.
13 jobs for !1579 with fd/lower-io-offsets-image-store-atomics in 23 minutes and 2 seconds (queued for 29 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #472111
debian

00:03:37

 
  Build+Test
passed #472120
meson-arm64

00:02:24

passed #472119
meson-armhf

00:01:49

passed #472113
meson-clang

00:09:29

passed #472117
meson-clover

00:13:51

passed #472121
meson-i386

00:01:12

passed #472116
meson-main

00:05:37

passed #472112
meson-swr-glvnd

00:06:34

passed #472118
meson-vulkan

00:02:06

passed #472123
scons-llvm

00:03:57

passed #472122
scons-nollvm

00:02:44

passed #472114
scons-swr

00:07:05

passed #472115
scons-win64

00:06:22