drm/bridge: synopsys/dw-hdmi: Enable cec clock

Support the "cec" optional clock. The documentation already mentions "cec"
optional clock and it is used by several boards, but currently the driver
doesn't enable it, thus preventing cec from working on those boards.

And even worse: a /dev/cecX device will appear for those boards, but it
won't be functioning without configuring this clock.

Changes:
v4:
- Change commit message to stress the importance of this patch

v3:
- Drop useless braces

v2:
- Separate ENOENT errors from others
- Propagate other errors (especially -EPROBE_DEFER)

Signed-off-by: Pierre-Hugues Husson <phh@phh.me>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20171125201844.11353-1-phh@phh.me