drm/exynos: mixer: enable NV12MT support for the video plane

The video processor supports a tiled version of the NV12 format,
known as NV12MT in V4L2 terms. The support was removed in commit
083500ba due to not being a real
pixel format, but rather NV12 with a special memory layout.

With the introduction of FB modifiers, we can now properly support
this format again.

Tested with a hacked up modetest from libdrm's test suite on
an ODROID-X2 (Exynos4412).

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>