drm/gem_shmem: Use a writecombine mapping for ->vaddr

Right now, the BO is mapped as a cached region when ->vmap() is called
and the underlying object is not a dmabuf.
Doing that makes cache management a bit more complicated (you'd need
to call dma_map/unmap_sg() on the ->sgt field everytime the BO is about
to be passed to the GPU/CPU), so let's map the BO with writecombine
attributes instead (as done in most drivers).

Fixes: 2194a63a

 ("drm: Add library for shmem backed GEM objects")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190529065121.13485-1-boris.brezillon@collabora.com