avcodec/libvpxenc: Adds support for constant quality mode in VP9.

Changes in the parameter mapping for libvpx to support the constant
quality mode in VP9. The assumption in the patch is that if crf is
provided but bitrate is 0, then the 'constant quality' mode of VP9
is used. However if both are present, the 'constrained quality' mode
is used as before.

Reviewed-by: James Zern <jzern@google.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>