intel: fix subslice computation from topology data

We're missing the offset of the slice in the subslice mask...

This worked for most platforms that don't have first slice fused off
because we would reread the same mask from slice0 again and again...

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Fixes: c1900f5b ("intel: devinfo: add helper functions to fill fusing masks values")
Gitlab: https://gitlab.freedesktop.org/mesa/mesa/issues/1869


Reviewed-by: Mark Janes <mark.a.janes@intel.com>
(cherry picked from commit d36763b2)