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>cc: @gerddie