drm/i915/guc: Don't go bang in GuC log if no GuC

If the GuC has failed to load for any reason and then the user pokes
the debugfs GuC log interface, a BUG and/or null pointer deref can
occur. Don't let that happen.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211210044022.1842938-5-John.C.Harrison@Intel.com