drm/atomic: Fix typo in kerneldoc

The for_each_crtc_in_state() is used to iterate over CRTCs rather than
connectors.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901144042.6023-2-thierry.reding@gmail.com