freedreno/layout: fix explicit layout offset not added to slice offset
Accidentally broke this when rebasing the offending commit.
My use case with non-zero explicit offset is UV plane of UBWC NV12, and only the UBWC slice offset is used for the UBWC sampler, so I didn't catch it immediately.
Fixes: d53dc6c3 (freedreno/fdl6: rework layout code a bit (reduce linear align to 64 bytes)
)