drm: Fix spelling of clock in drm_connector.h

Signed-off-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: trivial@kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1481894663-4570-1-git-send-email-pmeerw@pmeerw.net