drm/tegra: dc - Add YUYV support

YUYV is UYVY with swapped bytes. Luckily the Tegra DC hardware can swap
bytes during scan-out, so supporting YUYV is simply a matter of writing
the correct value to the byteswap register.

This patch modifies tegra_dc_format() to return the byte swap parameter
via an output parameter in addition to returning the pixel format. Many
other formats can potentially be supported in a similar way.

Signed-off-by: Thierry Reding <treding@nvidia.com>