drm/i915/guc: Process all G2H message at once in work queue

Rather than processing 1 G2H at a time and re-queuing the work queue if
more messages exist, process all the G2H in a single pass of the work
queue.

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210909164744.31249-6-matthew.brost@intel.com