glsl/linker: create a helper function to validate explicit locations

glsl/linker: create a helper function to validate explicit locations

Currently, we only validate explicit locations for non-SSO programs.
This creates a helper that we can call from both SSO and non-SSO paths
directly, so we can reuse all the logic behind this.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>