OMAPDSS: OMAP5: fix digit output's allowed mgrs

The OMAP5 HW supports directing DIGIT channel to DPI output, but the
driver doesn't support that. However, we have marked that configuration
as possible in the dss features, so in certain cases the driver tries to
use that configuration, leading to broken display.

Fix the problem by allowing DIGIT channel to go only to HDMI output.

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