drm/virtio: Use drm_gem_plane_helper_prepare_fb()

Use drm_gem_plane_helper_prepare_fb() helper for explicit framebuffer
synchronization. We need to wait for explicit fences in a case of
Venus and native contexts when guest user space uses explicit fencing.

Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>
[dmitry.osipenko@collabora.com: edit commit message]
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Acked-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241020230803.247419-1-dmitry.osipenko@collabora.com