drm/i915/guc: Cleanup adding GuC work items

We can just operate on the wq_tail directly (in the process descriptor).
This allows us to remove the duplicated tail from the client. While I'm
here let's also remove the constants kept in the client and document our
locking requirements. This causes a small change in one of GuC debugfs
files. We're no longer reporting constant values (which I don't think
is a problem), but we're also no longer reporting the tail (does anyone
care?).

v2: Update tail after wqi contents. (Chris)
v3: Really update tail after wqi contents.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170918092536.12287-1-michal.winiarski@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>