gpu: host1x: Add host1x_channel_stop()

Add host1x_channel_stop() which waits till channel becomes idle and then
stops the channel hardware. This is needed for supporting suspend/resume
by host1x drivers since the hardware state is lost after power-gating,
thus the channel needs to be stopped before client enters into suspend.

Tested-by: Peter Geis <pgwipeout@gmail.com> # Ouya T30
Tested-by: Paul Fertser <fercerpav@gmail.com> # PAZ00 T20
Tested-by: Nicolas Chauvet <kwizart@gmail.com> # PAZ00 T20 and TK1 T124
Tested-by: Matt Merhar <mattmerhar@protonmail.com> # Ouya T30
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>