glsl: do not try to reserve explicit locations for buffer variables

glsl: do not try to reserve explicit locations for buffer variables

Explicit locations are only used with uniform variables.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Marta Lofstedt <marta.lofstedt@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>