anv: advertise v6 of the wayland surface extension

anv: advertise v6 of the wayland surface extension

Jason updated the Khronos spec to explicitly state that Wayland surfaces
must support VK_PRESENT_MODE_MAILBOX_KHR.

ANV did so since day one (back in 2015)

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 43c188f9708b3e80b9f1c9c4c6bb16ac94b5ce5e)
[Emil Velikov: resolve trivial conflicts]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Conflicts:
	src/intel/vulkan/anv_device.c