r600/sfn: Initialize TestInstrFromString member m_instr_factory.

r600/sfn: Initialize TestInstrFromString member m_instr_factory.

Fix defect reported by Coverity Scan.

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

Fixes: e840645bb79 ("r600/sfn:explicitly initialize the memory pool")
Suggested-by: Gert Wollny <gert.wollny@collabora.com>
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/19098>