OMAPDSS: Add support for DPI source selection

We can select the video source for DPI output as follows:

OMAP2/3: always LCD1
OMAP4: LCD2 or DIGIT
OMAP5: LCD1/LCD2/LCD3/DIGIT

This patch adds support to select the source, and makes dpi.c call the
function to set the source.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: David Anders <x0132446@ti.com>