drm/i915: Set BXT cdclk to minimum initially

In case the driver is initialized without active displays, we should
just drop the cdclk to the minimum frequency right off the bat. There
might not be a modeset to drop it to the minimum late rafter all.

With DMC supposedly we should always have the cdclk up and running.
The DMC will shut the DE PLL down when appropriate, so let's nuke
the related FIXMEs as well. Trying to do anything different would
go against the expectations of the DMC firmware, and we all know
how fragile the DMC firmware is.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463172100-24715-22-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>