drm/i915/guc: init GuC descriptors after GuC load

GuC stores some data in there, which might be stale after a reset.
We already reset the WQ head and tail, but more things are being moved
to the descriptor with the interface updates. Instead of trying to track
them one by one, always memset and init the descriptors from scratch
after GuC is loaded.
The code is also reorganized so that the above operations and the
doorbell creation are grouped as "client enabling"

v2: add proc_desc_fini for symmetry (Daniele), remove unneeded var init,
add guc_is_alive() (Michal)

Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181002215430.15049-1-daniele.ceraolospurio@intel.com