drm/i915: s/gen8_setup_page_directory_pointer/gen8_setup_pml4e/

The function name gen8_setup_page_directory_pointer is misleading, and
only serves to confuse the reader, it's not setting up a pdp, but
rather encoding a specific pml4e with a given pdp.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>