drm: bridge/dw_hdmi: separate VLEVCTRL settting into platform driver

Because of iMX6 & Rockchip have differnet mpll config parameter,
the VLEVCTRL parameter would be different. In this case we should
separate VLEVCTRL setting from the common dw_hdmi driver, config
this parameter in platform driver(dw_hdmi-imx and dw_hdmi-rockchip)

Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>