radv/gfx9: add 3d sampler image->buffer copy shader. (v3)

On GFX9 we must access 3D textures with 3D samplers AFAICS.

This fixes:
dEQP-VK.api.image_clearing.core.clear_color_image.3d.single_layer

on GFX9 for me.

v1.1: fix tex->sampler_dim to dim
v2: send layer in from outside
v3: don't regress on pre-gfx9

Fixes: e38685cc 'Revert "radv: disable support for VEGA for now."'
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Tested-by: Alex Smith <asmith@feralinteractive.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
(cherry picked from commit a99fa7e8)