drm/i915: Reclaim GTT space for failed PPGTT

When the PPGTT init fails, we may as well reuse the space that we were
reserving for the PPGTT PDEs.

This also fixes an extraneous mutex_unlock.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>