OMAPDSS: DPI: fix dpi_get_dsidev() for omap5

On OMAP5 the DISPC channels and DSI PLLs are not connected the same way.
LCD2 on OMAP5 cannot use any DSI PLL as a source clock, but LCD3 can use
DSI2's PLL.

This patch fixes dpi_get_dsidev() by adding separate case for OMAP5 to
handle the difference.

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