drm/nouveau/disp: fix incorrect/broken hdmi methods

These are fixes from Lyude, and were meant to have been included in the
last round of drm-next patches.

- Fix some nasty memory issues that broke Lyude's display:
  - 0 initialize both nvif args and parsed HDMI infoframe buffers
  - Fixed missing memset(…, 0, …) for nvif args before sending VSI
    infoframe
  - Fixed incorrect data pointer and size in nvkm_uoutp_mthd_infoframe()
    (was previously pointing at the start of the nvif_outp_infoframe_args
    struct instead of at the start of the infoframe data
- Get rid of duplicated scdc assignments, since we only use it to write the
  scdc registers

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>