drm/omap: Fix crash when using LCD3 overlay manager

The channel_names list didn't have a string populated for LCD3 manager, this
results in a crash when the display's output is connected to LCD3. Add an entry
for LCD3.

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