nouveau: add interfaces to query information about supported classes

This will expose functionality supported by newer kernel interfaces.

Current userspace uses the chipset to determine which classes are likely
exposed, which generally works pretty well, but isn't as flexible as it
could be.

Unfortunately, the G98:GF100 video code in Mesa is still relying on the
kernel exposing incorrect vdec classes on some chipsets.  The ABI16
kernel interfaces have a workaround for this in place, but that will no
longer be available once libdrm supports NVIF.

To prevent a regression when NVIF support is added, if there's no kernel
support for NVIF, libdrm will magic up a class list containing correct
vdec classes anyway instead of failing with -ENODEV.

v2.
- add description of abi16/vdec workaround
- add description of sclass/mclass
- leave client-provided pointer unmodified on abi16_sclass() failure

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