drm/i915/chv: Freq(opcode) request for CHV.

On CHV, All the freq request should be even. So, we need to make sure we
request the opcode accordingly.

v2: Avoid vairable for freq request (ville)

Signed-off-by: Deepak S <deepak.s@linux.intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>