drm/amd/display: Remove redundant definition of dwb_source enums

There are repeated (but guarded) definitions of dwb_src enums. There are
also unused entires. Clean them up.

Signed-off-by: Julian Parkin <julian.parkin@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>