drm/i915: populate mem_freq/cz_clock for chv

We need mem_freq or cz clock for freq/opcode conversion

Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>