drm/atomic: Fix outdated comment.

Commit 0853695c ("drm: Add reference counting to drm_atomic_state")
adds reference counting to atomic state, but didn't update the comments
in drm_atomic_(nonblocking_)commit. Clarify lifetime a bit more.

Fixes: 0853695c ("drm: Add reference counting to drm_atomic_state")
Cc: <drm-intel-fixes@lists.freedesktop.org> # v4.10-rc1+
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/7acd1c78-ea86-7776-d98d-c846186e4b88@linux.intel.com