drm/i915: Update CDCLK_FREQ register on BDW after changing cdclk frequency

drm/i915: Update CDCLK_FREQ register on BDW after changing cdclk frequency

Update CDCLK_FREQ on BDW after changing the cdclk frequency. Not sure
if this is a late addition to the spec, or if I simply overlooked this
step when writing the original code.

This is what Bspec has to say about CDCLK_FREQ:
"Program this field to the CD clock frequency minus one. This is used to
 generate a divided down clock for miscellaneous timers in display."

And the "Broadwell Sequences for Changing CD Clock Frequency" section
clarifies this further:
"For CD clock 337.5 MHz, program 337 decimal.
 For CD clock 450 MHz, program 449 decimal.
 For CD clock 540 MHz, program 539 decimal.
 For CD clock 675 MHz, program 674 decimal."

Cc: stable@vger.kernel.org
Cc: Mika Kahola <mika.kahola@intel.com>
Fixes: b432e5cfd5e9 ("drm/i915: BDW clock change support")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461689194-6079-2-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Mika Kahola <mika.kahola@intel.com>