r600/sfn: Initialize FragmentShaderFromNir member m_pos_input.

r600/sfn: Initialize FragmentShaderFromNir member m_pos_input.

Fix defect reported by Coverity Scan.

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

Fixes: 374bc76706e ("r600/sfn: Add the position input as varying")
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/9004>