drm/i915/guc: pass request (not client) to i915_guc_{wq_check_space, submit}()

The knowledge of how to derive the relevant client from the request
should be localised within i915_guc_submission.c; the LRC code shouldn't
have to know about the internal details of the GuC submission process.
And all the information the GuC code needs should be encapsulated in (or
reachable from) the request.

v2:
    GEM_BUG_ON() for bad GuC client (Tvrtko Ursulin).
    Add/update kerneldoc explaining check_space/submit protocol

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>