mesa: Add spec citations for DispatchCompute*

mesa: Add spec citations for DispatchCompute*

Note: The OpenGL 4.3 - 4.5 specification language for DispatchCompute
appears to have an error regarding the max allowed values. When adding
the specification citation, we note why the code does not match the
specification language.

v2:
 * Updates based on review from Iago

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Cc: Iago Toral Quiroga <itoral@igalia.com>
Cc: Marta Lofstedt <marta.lofstedt@intel.com>
Reviewed-by: Marta Lofstedt <marta.lofstedt@intel.com>