drm/i915/bxt: Sanitize CDCLK to fix breakage during S4 resume

I noticed that during S4 resume BIOS incorrectly sets bits 18, 19 which
are reserved/MBZ and sets the decimal frequency fields to all 0xff in
the CDCLK register. The result is a hard lockup as display register
accesses are attempted later. Work around this by sanitizing the CDCLK
PLL/dividers the same way it's done on SKL.

While this is clearly a BIOS bug which should be fixed separately, it
doesn't hurt to check/sanitize this regardless.

v2:
- Use the same condition for VCO and CDCLK in broxton_init_cdclk as is
  used in skl_init_cdclk for the same purpose.

CC: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464093513-16258-2-git-send-email-imre.deak@intel.com