doc-rst: migrate ioctl CEC_DQEVENT to c-domain

This is only one example, demonstrating the benefits of the patch
series.  The CEC_DQEVENT ioctl is migrated to the sphinx c-domain and
referred by ":name: CEC_DQEVENT".

With this change the indirection using ":ref:`CEC_DQEVENT` is no longer
needed, we can refer the ioctl directly with ":c:func:`CEC_DQEVENT`". As
addition in the index, there is a entry "CEC_DQEVENT (C function)".

Signed-off-by: Markus Heiser <markus.heiser@darmarIT.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>