panvk: Fix assert
What does this MR do and why?
panvk: Fix assert
Fix defect reported by Coverity Scan.
Assign instead of compare (PW.ASSIGN_WHERE_COMPARE_MEANT)
assign_where_compare_meant: use of "=" where "==" may have been intended
Fixes: 0e74b6eda9a0 ("panvk: Add support for layered rendering")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>