drm/nouveau/fifo: set nvkm_engn_cgrp_get storage-class-specifier to static

smatch reports
drivers/gpu/drm/nouveau/nvkm/engine/fifo/runl.c:33:18:
  warning: symbol 'nvkm_engn_cgrp_get' was not declared. Should it be static?

nvkm_engn_cgrp_get is only used in runl.c, so it should be static

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230228221533.3240520-1-trix@redhat.com