u_blitter: do error checking assertions for shader caching

u_blitter: do error checking assertions for shader caching

If the user calls util_blitter_cache_all_shaders() set a flag and assert
that we never try to create any new fragment shaders after that point.
If the assertions fails, it means we missed generating some shader in
util_blitter_cache_all_shaders().

Reviewed-by: Marek Olšák <marek.olsak@amd.com>