drm/i915/display: move clk off sanitize to its own function

This allows us to isolate reading and writing to the
ICL_DPCLKA_CFGCR0 during the sanitize phase.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Clinton Taylor <Clinton.A.Taylor@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191217230529.25092-1-lucas.demarchi@intel.com