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 <[email protected]>
Cc: Anusha Srivatsa <[email protected]>
Cc: Rodrigo Vivi <[email protected]>
Cc: Imre Deak <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Gabriel Krisman Bertazi <[email protected]>
Reviewed-by: Michal Wajdeczko <[email protected]>