drm/nouveau/kms/nv50-: fix drm-get-put.cocci warnings

 Use drm_*_get() and drm_*_put() helpers instead of drm_*_reference() and
 drm_*_unreference() helpers.

Generated by: scripts/coccinelle/api/drm-get-put.cocci

Fixes: 30ed49b5 ("drm/nouveau/kms/nv50-: move code underneath dispnv50/")
Signed-off-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>