drm/nvc0-/gr: share headers between fermi and kepler graphics code

v2: Ben Skeggs <bskeggs@redhat.com>
- de-inline nv_icmd, triggers some gcc issue causing ctxnv[ce]0.c to
  take a *very* *very* long time to build on some configs.

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