gallium/st: add NV21 lowering to PIPE_FORMAT_G8_B8R8_420
This patch adds the possibility of lowering NV21 to G8_B8R8_420, by using the existing CSC in mesa, but letting the hardware do the texture sampling directly.
Previously, commit 826a1025 added a similar lowering from NV12 to R8_G8B8_420.
I'm not sure whether the lowered format should really be G8_B8R8, to me R8_B8G8 makes more sense, given that NV12 is being lowered to R8_G8B8.