drm/omap: Add pclk setting case when channel is DSS_WB

In dispc_set_ovl_common() we need to initialize pclk to a valid
value when we use WB in capture mode (i.e. mem_2_mem is false).
Otherwise dispc_ovl_calc_scaling() fails.

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