drm/msm/dpu: Revise _dpu_plane_get_aspace

Remove unneeded checks from _dpu_plane_get_aspace.

v3: change _dpu_plane_get_aspace to return a struct
*msm_gem_address_space instead passing in a pointer of the same
type to edit. Remove uneeded arguments.

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