drm/i915/selftests: fix error handling in __live_lrc_indirect_ctx_bb()

If intel_context_create() fails then it leads to an error pointer
dereference.  I shuffled things around to make error handling easier.

Fixes: 1dd47b54

 ("drm/i915: Add live selftests for indirect ctx batchbuffers")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200429132425.GE815283@mwanda