r600/sfn: Initialize VertexStageExportForGS m_num_clip_dist member variable.

r600/sfn: Initialize VertexStageExportForGS m_num_clip_dist member variable.

Fix warning reported by Coverity Scan.

Uninitialized scalar field (UNINIT_CTOR)
uninit_member: Non-static class member m_num_clip_dist is not
initialized in this constructor nor in any functions that it calls.

Fixes: f7df2c57a207 ("r600/sfn: extract class to handle the VS export to different stages")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5180>