OMAPDSS: explicitely initialize dssdev->channel for new displays

HDMI and VENC outputs always use the DIGIT output from DISPC. The dssdev
struct contains "channel" field which is used to specify the DISPC
output for the display, but this was not used for HDMI and VENC.

This patch fills the channel field explicitely for HDMI and VENC
displays so that we can always rely on the channel field.

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