anv/pipeline: set active_stages early

anv/pipeline: set active_stages early

Since the intermediate states of active_stages are not used,
i.e. active_stages is read only after all stages were set into it,
just set its value before compiling the shaders.

This will allow to conditionally run certain passes based on what
other shaders are being used, e.g. a certain pass might only be
applicable to the vertex shader if there's no geometry or tessellation
shader being used.

v2: Use vk_to_mesa_shader_stage. (Lionel)
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>