drm/i915/gvt/scheduler: Remove unnecessary NULL checks in sr_oa_regs

The checks are misleading and not required [1].

[1] https://lkml.org/lkml/2018/3/19/1792



Addresses-Coverity-ID: 1466017
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>