OMAPDSS: DISPC: pclk & lclk rates for writeback

Change the dispc_plane_pclk_rate and dispc_plane_lclk_rate functions to
return 0 if the given plane is the writeback plane. The clocks are not
valid for WB, but returning 0 from these functions instead of running
into BUG() will simplify the code that uses these functions.

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