drm: remove DRM_FORMAT_NV12MT

So this has been merged originally in

commit 83052d4d
Author: Seung-Woo Kim <sw0312.kim@samsung.com>
Date:   Thu Dec 15 15:40:55 2011 +0900

    drm: Add multi buffer plane pixel formats

which hasn't seen a lot of review really. The problem is that it's not
a real pixel format, but just a different way to lay out NV12 pixels
in macroblocks, i.e. a tiling format.

The new way of doing this is with the soon-to-be-merged fb modifiers.

This was brough up in some long irc discussion around the entire
topic, as an example of where things have gone wrong. Luckily we can
correct the mistake:
- The kms side support for NV12MT is all dead code because
  format_check in drm_crtc.c never accepted NV12MT.
- The gem side for the gsc support doesn't look better: The code
  forgets to set the pixel format and makes a big mess with the tiling
  mode bits, inadvertedly setting them all.

Conclusion: This never really worked (at least not in upstream) and
hence we can safely correct our mistake here.

Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Rob Clark <robclark@freedesktop.org>
Cc: Daniel Stone <daniel@fooishbar.org>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Acked-by: Joonyoung Shim <jy0922.shim@samsung.com>
Acked-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>