iris: always include an extra constbuf0 if using UBOs

iris: always include an extra constbuf0 if using UBOs

In st_nir_lower_uniforms_to_ubo() all UBO access in the shader have
its index incremented to open room for uniforms in constbuf0.  So if
we use UBOs, we always need to include the extra binding entry in the
table.

To avoid doing this checks both when compiling the shader and when
assigning binding tables, store the num_cbufs in iris_compiled_shader.

Fixes a bunch of tests from Piglit and CTS that use UBOs but don't use
uniforms or system values.  Note that some tests fitting this criteria
were passing because the UBOs were moved to be push
constants (avoiding the problem).

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>