drm/i915: Silence compiler for csr_load_work_fn()

drm/i915: Silence compiler for csr_load_work_fn()

gcc-4.7 is not very smart and can not tell that "si" is guarded by size
being 0. So it complains,

drivers/gpu/drm/i915/intel_csr.c: In function ‘csr_load_work_fn’:
drivers/gpu/drm/i915/intel_csr.c:204:3: warning: ‘si’ may be used uninitialized in this function [-Wmaybe-uninitialized]
drivers/gpu/drm/i915/intel_csr.c:190:30: note: ‘si’ was declared in

Give in and mark si as NULL.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171107145334.27154-1-chris@chris-wilson.co.uk
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>