OMAPDSS: Fix DSI_FCLK clock source selection

The wrong bit field was being updated in DSS_CTRL when trying to configure the
clock source of DSI2 functional clock. Use the correct bit field based on the
dsi module number.

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