drm/i915: Make bxt_set_cdclk() operate in terms of the current vs target DE PLL vco

Make bxt_set_cdclk() more readable by looking at current vs. target
DE PLL vco to determine if the DE PLL needs disabling and/or enabling.
We can also calculate the CD2X divider simply as (vco/cdclk) instead of
depending on magic numbers.

The magic numbers are still needed though, but only to map the supported
cdclk frequencies to corresponding DE PLL frequencies.

Note that w'll now program CDCLK_CTL correctly even for the bypass case.
Actually the CD2X divider should not matter in that case since the
hardware will bypass it too, but the "decimal" part should matter (if we
want to do gmbus/aux with the bypass enabled).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463172100-24715-20-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>