i965: fix prog_data leak in brw_disk_cache

i965: fix prog_data leak in brw_disk_cache

   ==25481== 576 bytes in 1 blocks are definitely lost in loss record 179 of 208
   ==25481==    at 0x4C2FB6B: malloc (vg_replace_malloc.c:299)
   ==25481==    by 0x1404E2CC: ralloc_size (ralloc.c:121)
   ==25481==    by 0x14119F82: read_and_upload (brw_disk_cache.c:176)
   ==25481==    by 0x1411A5C9: brw_disk_cache_upload_program (brw_disk_cache.c:271)
   ==25481==    by 0x1412FCA4: brw_upload_wm_prog (brw_wm.c:597)

Fixes: 516d50db319 ("i965: add initial implementation of on disk shader cache")
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>