drm/i915/guc: Add an explicit 'submission_initialized' flag

The LRC descriptor pool is going away. So, stop using it as a check
for whether submission has been initialised or not.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220302003357.4188363-3-John.C.Harrison@Intel.com