drm/i915: Split out Haswell code from gen6_pte_encode.

Now that we have function pointers, it's cleaner to just create a new
per-platform PTE encoding function.

This should be identical in behavior to the previous code.

v2: Drop accidental inline keyword on hsw_pte_encode.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Tested-by: Daniel Leung <daniel.leung@linux.intel.com> [v1]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>