drm/nouveau/dispnv04: Remove runtime PM

Originally when trying to fix the issue of runtime PM references with
non-blocking CRTCs on nv50, I ended up stumbling on this code when
trying to remove nouveau_drm->have_disp_power_ref, and attempted to fix
it to remove the dependency on have_disp_power_ref. However, Ilia Mirkin
pointed out that this code is actually completely useless, as pre-nv50
never had runtime PM support in the first place! Go figure.

So, since it's useless just get rid of it. Note that since the only
thing nouveau_crtc_set_config() was doing was grabbing a runtime PM ref,
calling drm_crtc_helper_set_config() then dropping the ref; we can just
remove the function entirely and just call drm_crtc_helper_set_config()
directly.

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