drm/atomic-docs: Spelling fixups

Eric nicely pointed these out, but I failed at git add and lost them.
This fixes up

commit 2f196b7c4b82eeff3574eb2999e78add33ef4361
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Jun 2 16:21:44 2016 +0200

    drm/atomic: Add drm_atomic_crtc_state_for_each_plane_state

to actually do what it says on the tin^Wcommit message.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

include/drm/drm_atomic.h

@@ -124,9 +124,9 @@ drm_atomic_get_existing_connector_state(struct drm_atomic_state *state,
  *
  * Note that this function is in general unsafe since it doesn't check for the
  * required locking for access state structures. Drivers must ensure that it is
- * save to access the returned state structure through other means. One commone
+ * safe to access the returned state structure through other means. One common
  * example is when planes are fixed to a single CRTC, and the driver knows that
- * the CRTC locks is held already. In that case holding the CRTC locks gives a
+ * the CRTC lock is held already. In that case holding the CRTC lock gives a
  * read-lock on all planes connected to that CRTC. But if planes can be
  * reassigned things get more tricky. In that case it's better to use
  * drm_atomic_get_plane_state and wire up full error handling.