OMAPDSS: DISPC: fix wb_setup issue for omap5

Currently we are trying to setup a overlay plane for WB which does not
exists. This is throughing a null pointer exception. This piece of code
was never validated on OMAP5.

Change-Id: I3fd28a60d59fd8c30904423b15eecd1a4cc3dc0b
Signed-off-by: Dandawate Saket <dsaket@ti.com>