mesa: add missing return statement for GL_RG_SNORM case

Fixes: 0d356cf4781bece0dc9a7 "mesa: enable EXT_render_snorm extension"
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
1 file changed