drm/i915/guc: Fix a memory leak where guc->execbuf_client is not freed

During driver unloading, the guc_client created for command submission
needs to be released to avoid memory leak.

The struct_mutex needs to be held before tearing down GuC.

v1: Move i915_guc_submission_disable out of i915_guc_submission_fini and
    take struct_mutex lock before release GuC client. (Dave Gordon)
v2: Add the locking for failure case in guc_fw_fetch. (Dave Gordon)
    Add i915_guc_submission_fini for failure case in intel_guc_ucode_load.

Signed-off-by: Alex Dai <yu.dai@intel.com>
Reviewed-by: Dave Gordon <david.s.gordon@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452711710-4505-1-git-send-email-yu.dai@intel.com