drm/i915/guc: Simplify guc client

We originally added support, in some cases partial, for different modes
of operations via guc clients:

- proxy vs direct submission;
- variable engine mask per-client.

We only ever used one flow (all submissions via a single proxy), so the
other code paths haven't been exercised and are most likely
non-functional. The guc firmware interface is also in the process of
being updated to better fit the i915 flow and our client abstraction
will need to change accordingly (or possibly go away entirely), so these
old unused paths can be considered dead and removed.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Acked-by: Matthew Brost &lt;Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190710005437.3496-3-daniele.ceraolospurio@intel.com