OMAPDSS: DSI: use dsi_get_dsidev_id(dsidev) instead of dsidev->id

The DSI driver uses dsi_get_dsidev_id() to get the ID number for the DSI
instance. However, there were a few places where dsidev->id was used
instead of the function. Fix those places to use the function.

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