drm/i915/cdclk: prefer intel_de_write() over I915_WRITE()

Let's try to not add new ones while we're phasing out I915_READ() and
I915_WRITE().

Fixes: 27a6bc80

 ("drm/i915/dg1: Initialize RAWCLK properly")
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130111601.2817-3-jani.nikula@intel.com