drm/i915/selftests: Skip live context execution test without logical contexts

If the HW (or driver) doesn't support logical contexts, don't pretend we
gain anything from trying to execute GPU commands with them. At best it
reports -ENODEV, which is an unhelpful failure that we should just skip.

v2: Be more specific and check the driver/engine caps for logical (HW)
context support.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180706101923.28548-1-chris@chris-wilson.co.uk