zink: Remove duplicate variable zm.

zink: Remove duplicate variable zm.

Fix defect reported by Coverity Scan.

Evaluation order violation (EVALUATION_ORDER)
write_write_typo: In
zm = zm = create_shader_module_for_stage_optimal(ctx, screen, prog->shaders[i], prog, i, state),
zm is written twice with the same value.

Fixes: 325c703624a ("zink: add 'optimal_keys' handling for shader keys")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19086>