drm/nouveau/nvkm/subdev/mc/tu102: Make functions called by reference static

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/nouveau/nvkm/subdev/mc/tu102.c:50:1: warning: no previous prototype for ‘tu102_mc_intr_unarm’ [-Wmissing-prototypes]
 drivers/gpu/drm/nouveau/nvkm/subdev/mc/tu102.c:62:1: warning: no previous prototype for ‘tu102_mc_intr_rearm’ [-Wmissing-prototypes]
 drivers/gpu/drm/nouveau/nvkm/subdev/mc/tu102.c:74:1: warning: no previous prototype for ‘tu102_mc_intr_mask’ [-Wmissing-prototypes]

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: dri-devel@lists.freedesktop.org
Cc: nouveau@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210602143300.2330146-16-lee.jones@linaro.org