drm/i915/cnl: Remove spurious central_freq.

"Display software must leave this field at the default value.
It no longer needs to be configured as part of PLL programming."

We respect this already and we are setting up the default
one line below: "DPLL_CFGCR1_CENTRAL_FREQ".

Also we don't touch anywhere else this central_freq for cnl.
So let's remove from the final write.

No functional change. Only a clean-up patch.

Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Mika Kahola <mika.kahola@intel.com>
Cc: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114194759.24541-2-rodrigo.vivi@intel.com