drm/nouveau/nv50-: separate CHANNEL_GPFIFO handling out from CHANNEL_DMA

Primarily a cleanup to allow for changes in newer CHANNEL_GPFIFO classes
to be more easily implemented.

Compared to the prior implementation, this submits userspace push buffer
segments as subroutines and uses the NV_RAMUSERD_TOP_LEVEL_GET registers
to track the main (kernel) push buffer progress.

Fixes a number of sporadic failures seen during piglit runs.

Signed-off-by: Ben Skeggs <bskeggs@nvidia.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Timur Tabi <ttabi@nvidia.com>
Tested-by: Timur Tabi <ttabi@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>