drm/i915: call kunmap_px on pt_vaddr

We need to kunmap pt_vaddr and not pt itself, otherwise we end up
mapping a bunch of pages without ever unmapping them.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Fixes: d1c54acd ("drm/i915/gtt: Introduce kmap|kunmap for dma page")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460476663-24890-4-git-send-email-matthew.auld@intel.com