mesa/main: add support for ARB_compute_variable_groups_size

mesa/main: add support for ARB_compute_variable_groups_size

v5: - replace fixed_local_size by !LocalSizeVariable (Nicolai)
v4: - slightly indent spec quotes (Nicolai)
    - drop useless _mesa_has_compute_shaders() check (Nicolai)
    - move the fixed local size outside of the loop (Nicolai)
    - add missing check for invalid use of work group count
v2: - update formatting spec quotations (Ian)
    - move the total_invocations check outside of the loop (Ian)

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