drm/i915/guc: rework guc_add_workqueue_item()

Mostly little optimisations and future-proofing against code breakage.
For instance, if the driver is correctly following the submission
protocol, the "out of space" condition is impossible, so the previous
runtime WARN_ON() is promoted to a GEM_BUG_ON() for a more dramatic
effect in development and less impact in end-user systems.

Similarly we can make alignment checking more stringent and replace
other WARN_ON() conditions that don't relate to the runtime hardware
state with either BUILD_BUG_ON() for compile-time-detectable issues, or
GEM_BUG_ON() for logical "can't happen" errors.

With those changes, we can convert it to void, as suggested by Chris
Wilson, and update the calling code appropriately.

v2:
    Note that we're now putting the request seqno in the "fence_id"
    field of each GuC-work-item, in case it turns up somewhere useful
    (e.g. in a GuC log) [Tvrtko Ursulin].

Signed-off-by: Dave Gordon <david.s.gordon@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>