radeonsi: silent a compiler warning

radeonsi: silent a compiler warning

This fixes:

si_shader.c: In function ‘si_shader_dump_stats’:
si_shader.c:6704:31: warning: passing argument 1 of ‘si_get_max_workgroup_size’ discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
     si_get_max_workgroup_size(shader);
                               ^~~~~~
si_shader.c:5832:17: note: expected ‘struct si_shader *’ but argument is of type ‘const struct si_shader *’
 static unsigned si_get_max_workgroup_size(struct si_shader *shader)

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>