drm/gk20a/timer: initial implementation

A bit different from NVIDIA's RFC patch, but I want this now for GM107.

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

diff --cc drivers/gpu/drm/nouveau/Makefile
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/include/subdev/timer.h
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/subdev/timer/gk20a.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/subdev/timer/nv04.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/subdev/timer/nv04.h
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/subdev/timer/priv.h
Simple merge