gpu: nvgpu: gk20a: allow syncpt free at channel_unbind()

gpu: nvgpu: gk20a: allow syncpt free at channel_unbind()

Add infrastructure to allow freeing gpu channels' syncpts at
channel_unbind()

Currently, we free the syncpt at channel_free() only.
But we can free the syncpt when channel becomes idle.

When we do channel_unbind(), channel cannot be scheduled and
hence we can destroy its syncpt.
Channel will request again for new syncpt when it has new work
to do.

This feature will be enabled with boolean flag
syncpt_aggressive_destroy

Bug 1305024

Change-Id: Ib498e2c371e36ffc1430d4f95f2780b4c587e43e
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/395153
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Tested-by: Terje Bergstrom <tbergstrom@nvidia.com>