drm/amd/display: Small comment on dc_commit_planes_to_stream

Having one struct on the stack was somewhat confusing. Also mention
that the whole function should eventually go.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>