drm/omap: hdmi4_cec: Fix CEC clock handling for PM

If CONFIG_OMAP4_DSS_HDMI_CEC is enabled in .config, deeper SoC idle
states are blocked because the CEC clock gets always enabled on init.

Let's fix the issue by moving the CEC clock handling to happen later in
hdmi_cec_adap_enable() as suggested by Hans Verkuil <hverkuil@xs4all.nl>.
This way the CEC clock gets only enabled when needed. This can be tested
by doing cec-ctl --playback to enable the CEC, and doing cec-ctl --clear
to disable it.

Let's also fix the typo for "divider" in the comments while at it.

Fixes: 8d7f934d ("omapdrm: hdmi4_cec: add OMAP4 HDMI CEC support")
Suggested-by: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Cc: Jyri Sarha <jsarha@ti.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326151438.32414-1-tony@atomide.com