gpu: host1x: Remove unused host1x_cdma_stop() definition

There is no host1x_cdma_stop() in the code, let's remove its definition
from the header file.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Erik Faye-Lund <kusmabite@gmail.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>