ac: fix image load store for GLSL_SAMPLER_DIM_3D
Fixes the following piglit tests:
arb_shader_image_load_store/layer/image3d/layered binding test
arb_shader_image_load_store/max-size/image3d max size test/2048x8x8x1
arb_shader_image_load_store/max-size/image3d max size test/8x2048x8x1
arb_shader_image_load_store/max-size/image3d max size test/8x8x2048x1
arb_shader_image_load_store/semantics/imageload/vertex shader/rgba32f/image3d test
Reviewed-by:
Nicolai Hähnle <nicolai.haehnle@amd.com>