radeonsi: Count planes for imported textures.

radeonsi: Count planes for imported textures.

For the DRI2 lowered YUV import separate pipe_resources get created
but in the end the first resource just gets asked for NPLANES.

Since

1) (Almost) everything uses the first resource + a plane index in the
  Gallium interface.
2) This mirrors non-imported textures.

lets fix this in the driver.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4779>