st/glsl_to_tgsi: make use of driver_cache_blob with the disk cache

st/glsl_to_tgsi: make use of driver_cache_blob with the disk cache

driver_cache_blob was introduced with the i965 disk cache, it allows
us to simplify the cache a little and possibly offers some minor
speed improvements since we load the GLSL metadata and TGSI from
disk in one pass.

Using driver_cache_blob should also make it straight forward to
implement binary support for ARB_get_program_binary in gallium.

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