drm/nvc0/gr: import and use our own fuc by default

The ability to use NVIDIA's fuc has been retained *temporarily* in order
to better debug any issues that may be lingering in our initial attempt
at writing this ucode.  Once I'm fairly confident we're okay, it'll be
removed.

There's a number of things not implemented by this fuc currently, but
most of it is sets of state that our context setup would not have used
anyway.  No doubt we'll find out what they're for at some point, and
implement it if required.

This has been tested on 0xc0/0xc4 thus far, and from what I could tell
it worked as well as NVIDIA's.  It's also been tested on 0xc1, but even
with NVIDIA's fuc that chipset doesn't work correctly with nouveau yet.

0xc3/0xc8/0xce should in theory be supported too, but I don't have the
hardware to check that.

There's no doubt numerous bugs to squash yet, please report any!

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>