drm/i915/guc: Split guc_wq_item_append

drm/i915/guc: Split guc_wq_item_append

We're using a special preempt context for HW to preempt into. We don't
want to emit any requests there, but we still need to wrap this context
into a valid GuC work item.
Let's cleanup the functions operating on GuC work items.
We can extract guc_request_add - responsible for adding GuC work item and
ringing the doorbell, and guc_wq_item_append - used by the function
above, not tied to the concept of gem request.

Signed-off-by: Michał Winiarski <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Jeff McGee <[email protected]>
Cc: Michal Wajdeczko <[email protected]>
Cc: Oscar Mateo <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Reviewed-by: Jeff McGee <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]