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: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NBen Widawsky <ben@bwidawsk.net>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>