mesa/glsl: add cache_fallback flag to gl_shader_program_data

This will allow us to skip certain things when falling back to
a full recompile on a cache miss such as avoiding reinitialising
uniforms.

In this change we use it to avoid reading the program metadata
from the cache and skipping linking during a fallback.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>

src/compiler/glsl/shader_cache.cpp

@@ -1281,7 +1281,7 @@ shader_cache_read_program_metadata(struct gl_context *ctx,
       return false;
 
    struct disk_cache *cache = ctx->Cache;
-   if (!cache)
+   if (!cache || prog->data->cache_fallback)
       return false;
 
    /* Include bindings when creating sha1. These bindings change the resulting