OMAP: DSS2: VENC: don't call platform_enable/disable() twice

platform_enable/disable() is already called in venc_power_on/off(), so
don't do it again in venc_panel_enable/disable().

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>