drm/i915/gvt: Fix the kernel null pointer error

once error happens in shadow_indirect_ctx function, the variable
wa_ctx->indirect_ctx.obj is not initialized but accessed, so the
kernel null point panic occurs.

Fixes: 894cf7d1 ("drm/i915/gvt: i915_gem_object_create() returns an error pointer")
Cc: stable@vger.kernel.org # v4.8+
Signed-off-by: fred gao <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>