drm/i915/guc: Allow preempt-client to be NULL

drm/i915/guc: Allow preempt-client to be NULL

In the next patch, we may only conditionally allocate the preempt-client
if there is a global preempt context and so we need to be prepared in
case the preempt-client itself is NULL.

v2: Grep for more preempt_client.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180207210544.26351-1-chris@chris-wilson.co.uk
Reviewed-by: Michel Thierry <michel.thierry@intel.com>