v3d: YUV formats are supported as external-only dmabuf modifiers
This series fixes an Issue "Issue Raspberry 4-64 + Mesa VC4 driver + Gstreamer = red Label on video".
The last patch also avoids exposing DRM_FORMAT_MOD_BROADCOM_SAND128 for formats different to PIPE_FORMAT_NV12
To backport the second patch to 20.0 I'll create a different MR when it lands as there will be conflicts after 95c4f0f9 "v3d: Enables DRM_FORMAT_MOD_BROADCOM_SAND128 support" landed, so that is the reason this patch doesn't reference previous commit.