drm/nouveau/nvif: remove disp chan rd/wr

There's no good reason the ioremap() that results from nvif_object_map()
should fail, so add a check that the map succeeded, and remove the rd/wr
methods from display channel objects.

As this was the last user of rd/wr methods, the nvif plumbing is removed
at the same time.

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240726043828.58966-27-bskeggs@nvidia.com