drm/amd/display: pull commit_surfaces out of atomic_commit into helper function

This should make things simpler when we try to rework this later when we
pass validate_context from atomic_check to atomic_commit.

Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>