iris: Track bound constant buffers

This helps avoid having to iterate over [0, PIPE_MAX_CONSTANT_BUFFERS)
looking to see if any resources are bound.

src/gallium/drivers/iris/iris_context.h

@@ -322,6 +322,9 @@ struct iris_shader_state {
    struct iris_sampler_state *samplers[IRIS_MAX_TEXTURE_SAMPLERS];
    struct iris_sampler_view *textures[IRIS_MAX_TEXTURE_SAMPLERS];
 
+   /** Bitfield of which constant buffers are bound (non-null). */
+   uint32_t bound_cbufs;
+
    /** Bitfield of which image views are bound (non-null). */
    uint32_t bound_image_views;
 

src/gallium/drivers/iris/iris_state.c

@@ -2465,6 +2465,8 @@ iris_set_constant_buffer(struct pipe_context *ctx,
    struct iris_const_buffer *cbuf = &shs->constbuf[index];
 
    if (input && input->buffer) {
+      shs->bound_cbufs |= 1u << index;
+
       assert(index > 0);
 
       pipe_resource_reference(&cbuf->data.res, input->buffer);
@@ -2475,6 +2477,7 @@ iris_set_constant_buffer(struct pipe_context *ctx,
 
       upload_ubo_surf_state(ice, cbuf, input->buffer_size);
    } else {
+      shs->bound_cbufs &= ~(1u << index);
       pipe_resource_reference(&cbuf->data.res, NULL);
       pipe_resource_reference(&cbuf->surface_state.res, NULL);
    }