iris: move sysvals to their own constant buffer

This commit moves the sysvals to a separate, new constant buffer
at the end (before the shader constants). It also allows us to
remove the special handling we had for cbuf0, and enables all
constant buffers to support user-specified resources and user
buffers.

v2: (by Kenneth Graunke)
- Rebase on the previous patch to fix system value uploading.
- Fix disk cache num_cbufs calculation
- Fix passthrough TCS to report num_cbufs = 1 so upload actually occurs
- Change upload_sysvals to assert that num_cbufs > 0 when
  num_system_values > 0.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/gallium/drivers/iris/iris_context.h

@@ -357,8 +357,7 @@ struct iris_shader_state {
    struct pipe_shader_buffer constbuf[PIPE_MAX_CONSTANT_BUFFERS];
    struct iris_state_ref constbuf_surf_state[PIPE_MAX_CONSTANT_BUFFERS];
 
-   struct pipe_constant_buffer cbuf0;
-   bool cbuf0_needs_upload;
+   bool sysvals_need_upload;
 
    /** Shader Storage Buffers */
    struct pipe_shader_buffer ssbo[PIPE_MAX_SHADER_BUFFERS];