drm/amd/display: fix index and union overwrite in compressor

  Fixing 2 bugs in compressor:
- array out of bounds due to incorrect index
- compressor options always 0 due to union overwrite

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>