drm/nouveau: remove symlinks, move core/ to nvkm/ (no code changes)

The symlinks were annoying some people, and they're not used anywhere
else in the kernel tree.  The include directory structure has been
changed so that symlinks aren't needed anymore.

NVKM has been moved from core/ to nvkm/ to make it more obvious as to
what the directory is for, and as some minor prep for when NVKM gets
split out into its own module (virt) at a later date.

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