drm/bridge: cdns: Mark runtime PM operations as maybe unused

Building the driver in a configuration with !PM currently causes a
warning about these operations being unused. Mark them as such to shut
up the compiler.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180426135853.30895-1-thierry.reding@gmail.com