OMAPDSS: DISPC: Remove blocking code from dispc_wb_enable()

WB will not be used with compat-mode, i.e. from omapfb. This means we
don't need the current complex dispc_wb_enable function, but can have a
simple register write version of the function.

This patch removes all the extra code from dispc_wb_enable()

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