freedreno/layout: fix explicit layout offset not added to slice offset

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: d53dc6c37680eba8e8 ("freedreno/fdl6: rework layout code a bit (reduce linear align to 64 bytes)")
Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5761>