mesa/glsl: add cache_fallback flag to gl_shader_program_data

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>