drm/xe/vm: Use for_each_tlb_inval() to calculate invalidation fences

drm/xe/vm: Use for_each_tlb_inval() to calculate invalidation fences

ops_execute() calculates the size of a fence array based on
XE_MAX_GT_PER_TILE, while the code that actually fills in the fence
array uses a for_each_tlb_inval() iterator.  This works out okay today
since both approaches come up with the same number of invalidation
fences (2: primary GT invalidation + media GT invalidation), but could
be problematic in the future if there isn't a 1:1 relationship between
TLBs needing invalidation and potential GTs on the tile.

Adjust the allocation code to use the same for_each_tlb_inval()
counting logic as the code that fills the array to future-proof the
code.

Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patch.msgid.link/20251118202604.3715782-2-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>