drm/i915/skl: Don't try to store the wrong central frequency

The orignal code started by storing the actual central frequency (in Hz,
using a uint64_t) in a uint32_t which codes for the register value. That
can't be right.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>