OMAPDSS: Fix writes to DISPC_POL_FREQ

When omapdss writes to DISPC_POL_FREQ register, it always ORs the bits
with the current contents of the register, never clearing the old ones,
causing wrong signal polarity settings.

As we write all the bits in DISPC_POL_FREQ, we don't need to care about
the current contents of the register. So fix the issue by constructing
new register value from scratch.

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