OMAPDSS: DISPC: fix dispc_mgr_lclk_rate for DIGIT output

dispc_mgr_lclk_rate() cannot currently be called with DIGIT channel
parameter, even if dispc_ovl_lclk_rate() can. Fix this by making
dispc_mgr_lclk_rate() handle DIGIT channel also.

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