birefnet: switch to direct version of conv_2d_deform on Vulkan

* times old -> new / new with coopmat2
* birefnet: 268ms -> 315ms / 243ms
* birefnet-lite: 109ms -> 119 / 87ms
* deform conv2d is a bit slower without coopmat2 support, but also requires much less vram, so still worth it