anv: Disable transform feedback on gen7

It's totally implementable, it's just that the plumbing is a bit
different and we never hooked it up.  Don't advertise a broken feature.

Fixes: 36ee2fd6 "anv: Implement the basic form of VK_EXT_transform_feedback"
(cherry picked from commit 295e5a17)
[Juan A. Suarez: resolve trivial conflicts]
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>

Conflicts:
	src/intel/vulkan/anv_extensions.py