This function will be needed by the next patch where the driver
calculates the BW based on driver specific parameters, so export it.

At the same time sanitize the function params, passing the more natural
link rate instead of the encoding of the same rate.

v2:
- Fix function documentation. (Lyude)

Cc: Lyude Paul <lyude@redhat.com>
Cc: Ville Syrjala <ville.syrjala@intel.com>
Cc: <stable@vger.kernel.org>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NLyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210125173636.1733812-1-imre.deak@intel.com