drm/scdc-helper: Use consistent spelling for TMDS

The file uses inconsistent capitalization for TMDS. Since it is an
abbreviation, all uppercase is correct.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901144430.19164-3-thierry.reding@gmail.com