anv: Fix a relocation race condition

anv: Fix a relocation race condition

Previously, we would read the offset from the BO in anv_reloc_list_add
to generate the presumed offset and then again in the caller to compute
the 64-bit address to write into the buffer.  However, if the offset
somehow changed between these two points, the presumed offset would no
longer match the written offset.  This is unlikely to actually ever be a
problem in practice because the presumed offset gets recorded first and
so if the written address is wrong then the presumed offset is almost
certainly wrong and the relocation will trigger.  However, it's much
safer to simply have anv_reloc_list_add return the 64-bit address.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>