util/format: Use correct pipe format for VK_FORMAT_G8_B8_R8_3PLANE_420_UNORM.

util/format: Use correct pipe format for VK_FORMAT_G8_B8_R8_3PLANE_420_UNORM.

NV12 is UVUVUV (https://wiki.videolan.org/YUV#NV12) and in Vulkan is
VK_FORMAT_G8_B8R8_2PLANE_420_UNORM. So U=B and V=R. So plane order in
VK_FORMAT_G8_B8_R8_3PLANE_420_UNORM is YUV, which is PIPE_FORMAT_IYUV.

Further confirmation: https://fourcc.org/yuv.php U=Cb V=Cr. From the nir
ycbcr conversion, B=Cb and R=Cr.

Fixes: 75d7ee80291 "util/format: translate 422_UNORM and 420_UNORM vulkan formats"
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jonathan Marek <jonathan@marek.ca>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5195>