drm/vc4: Add support for NV21 and NV61.

NV12 (YUV420 2 plane) and NV16 (YUV422 2 plane) were
supported, but NV21 and NV61 (same but with Cb and Cr
swapped) weren't. Add them.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/1f50799525e3401551dff2b0b2828b9ab892f75f.1510841336.git.dave.stevenson@raspberrypi.org