nv50: fix g98+ vdec class allocation

nv50: fix g98+ vdec class allocation

The kernel previously exposed incorrect classes for some of the chipsets
that this code supports.  It no longer does, but the older object ioctls
have compatibility to avoid breaking userspace.

This needs to be fixed before switching over to the newer interfaces.

Rather than hardcoding chipset->class like the rest of the driver does,
this makes use of (new) sclass queries to determine what's available.

v2.
- update to use symbolic class identifier from <nvif/class.h>

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>