drm/i915: s/gen8_setup_page_directory/gen8_setup_pdpe/

The function name gen8_setup_page_directory is misleading, and only
serves to confuse the reader, it's not setting up a pd, but rather
encoding a specific pdpe with a given pd.

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>