drm: Update docs around gem_free_object

Not all places correctly stated that gem_free_object_unlocked is the
one to use.

Reported-by: Eric Anholt <eric@anholt.net>
Cc: Eric Anholt <eric@anholt.net
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170718063337.31942-1-daniel.vetter@ffwll.ch


Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>