drm/i915: don't open code the pdpe/pml4e clearing

Now that it's obvious what the helpers do, we can simplify the code
somewhat by using them when clearing the pdpe/pml4e with the relevant
scratch entry.

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: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161213160512.7008-1-matthew.auld@intel.com