zink: Check for VK_EXT_extended_dynamic_state3 before setting A2C

zink: Check for VK_EXT_extended_dynamic_state3 before setting A2C

Fix crashes for piglit alpha-to-coverage tests when
VK_EXT_extended_dynamic_state3 is not supported (like on Venus)

Fixes: 736577871b9 ("zink: check for cbuf0 writes before setting A2C")

Signed-off-by: Mary Guillemard <mary.guillemard@collabora.com>
Reviewed-by: Antonino Maniscalco <antonino.maniscalco@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26003>
(cherry picked from commit fe159c85de81a12e895342d003821aa4f25db8d5)