drm/i915/gem: Improve pfn calculation readability in vm_fault_gtt()

By moving the pfn calculation to the set_address_limits()
function we improve code readability. This way,
set_address_limits() is responsible for calculating all memory
mapping paramenters: "start", "end" and "pfn".

This suggestion from Jonathan was made during the review of
commit 8bdd9ef7 ("drm/i915/gem: Fix Virtual Memory mapping
boundaries calculation"), which I liked, but it got lost on the
way.

Suggested-by: Jonathan Cavitt <Jonathan.cavitt@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Krzysztof Niemiec <krzysztof.niemiec@intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240807104553.481763-1-andi.shyti@linux.intel.com