drm/i915/chv: Use 16 and 32 for low and high drain latency precision.

Current chv spec teels we can only use either 16 or 32 bits as precision.

Although in the past VLV went from 16/32 to 32/64 and spec might not be updated,
these precision values brings stability and fixes some issues Wayne was facing.

Cc: Wayne Boyer <wayne.boyer@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Tested-by: Wayne Boyer <wayne.boyer@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Sprinkle const as requested by Ville.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>