OMAPDSS: fix omap2 dss fck handling

The driver considers OMAP2 DSS's functional clock as a fixed clock.
However, it can be changed, but the possible dividers are not continuous
which is why it was just handled as a fixed clock.

As a partial fix, this patch changes the code to handle the continous
part of the dividers, from 1 to 6. This let's us handle the OMAP2 fck
the same way as fcks on other OMAPs.

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