This is needed by radeonsi to support multi plane modifier when
using modesetting DDX.
xserver reject any multi plane buffer (in DRI3PixmapFromBuffers)
without the dmabuf_capable debug option. It's OK for single plane
buffer as xserver will fallback to DRI3BufferFromPixmap path.
amdgpu DDX does not expose DRI3PixmapFromBuffers, mesa just use
the single plane path anyway, so amdgpu DDX does not have this
problem.
Reviewed-by: Sultan Alsawaf <sultan@kerneltoast.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2091>