drm/msm/dpu: Make dpu_plane_danger_signal_ctrl void

Removed all impossible checks from the function, which eliminates
the need for a return value. This function is also never used
outside of dpu_plane.c, so the function is made static.

v3: Using helper function _dpu_plane_get_kms() instead of doing
it locally.

Signed-off-by: Bruce Wang <bzwang@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>