drm: Drop the NV12M and YUV420M formats

The NV12M/YUV420M formats are identical to the NV12/YUV420 formats.
So just remove these duplicated format names.

This might look like breaking the ABI, but the code has never actually
accepted these formats, so nothing can be using them.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>