OMAPDSS: remove dispc's dependency to VENC/HDMI

DISPC needs to know the clock rate for DIGIT (i.e. TV) channel, and this
clock is provided by either VENC or HDMI modules. Currently DISPC will
call a function in VENC/HDMI, asking what the clock rate is. This means
we have a fixed dependency from DISPC to both VENC and HDMI.

To have a more generic approach, and in particular to allow adding OMAP5
HDMI driver, we need to remove this dependency. This patch makes
VENC/HDMI inform DISPC when the their clock changes, thus reversing the
dependency and removing the issue.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>