anv: Fix assert in xe_gem_create()
What does this MR do and why?
In this assert we want to enforce that if a cached buffer is created it is a cached+coherent as Xe KMD don't support cached+incoherent.
Did not caught this issue because it only reproduces in platforms with GPU outside of LLC.
Fixes: 9d8d5cf8 ("anv: Remove block promoting non CPU mapped bos to coherent")