drm/i915/gtt: Rename unmap_and_free_px to free_px

All the paging structures are now similar and mapped for
dma. The unmapping is taken care of by common accessors, so
don't overload the reader with such details.

v2: Be consistent with goto labels (Michel)

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>