drm/i915: Warn about odd rps values on CHV

CHV wants even rps opcodes so print a warning of the
min/max/rpe/rp1 values are odd, and warn if an odd value
slips through to valleyview_set_rps() and truncate it to
an even value.

Also add a comment to chv_freq_opcode() to make sure no one
changes the code without considering this requirement.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Deepak S <deepak.s@linux.intel.com>
[danvet: Help git along in applying the patch, somehow it silently
ended up in the vlv init_gt_powersave function.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>