i965: fix disk_cache leak when destroying context

i965: fix disk_cache leak when destroying context

   ==2780== 1,024 bytes in 1 blocks are possibly lost in loss record 180 of 205
   ==2780==    at 0x4C31A1E: calloc (vg_replace_malloc.c:711)
   ==2780==    by 0x13F6467E: util_queue_init (u_queue.c:309)
   ==2780==    by 0x13F5C9F6: disk_cache_create (disk_cache.c:369)
   ==2780==    by 0x13F05406: brw_disk_cache_init (brw_disk_cache.c:428)
   ==2780==    by 0x13F01E78: brwCreateContext (brw_context.c:1068)

Fixes: 1a61a8b9a7c ("i965: Initialize disk shader cache if MESA_GLSL_CACHE_DISABLE is false")
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>