drm/msm/mdp5: Prepare mdp5_pipe_assign for some rework

mdp5_pipe_assign currently returns the hwpipe pointer for the drm_plane.
Return it indirectly by setting a pointer passed as an argument. This
is needed because we want the func to find out the right hwpipe too.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>