drm/vc4: prepare for CEC support

In order to support CEC the hsm clock needs to be enabled in
vc4_hdmi_bind(), not in vc4_hdmi_encoder_enable(). Otherwise you wouldn't
be able to support CEC when there is no hotplug detect signal, which is
required by some monitors that turn off the HPD when in standby, but keep
the CEC bus alive so they can be woken up.

The HDMI core also has to be enabled in vc4_hdmi_bind() for the same
reason.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20170716104804.48308-3-hverkuil@xs4all.nl
(cherry picked from commit 10ee275c)