drm/gk20a: add BAR instance

GK20A's BAR is functionally identical to NVC0's, but do not support
being ioremapped write-combined. Create a BAR instance for GK20A that
reflect that state.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

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

diff --cc drivers/gpu/drm/nouveau/core/engine/device/nve0.c
Simple merge

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

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

diff --cc drivers/gpu/drm/nouveau/core/subdev/bar/nvc0.c
Simple merge

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