drm/i915/selftests: Avoid ERR_PTR dereference

drm/i915/selftests: Avoid ERR_PTR dereference

Along the early error path for igt_switch_to_kernel_context we may try
to dereference an invalid error pointer. Instead, return early rather
than dump the GEM trace since we haven't yet emitted anything of
interest.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 06b81dac2abf ("drm/i915: Look for an active kernel context before switching")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180620112441.13085-1-chris@chris-wilson.co.uk