drm/i915: factor out is_always_on_domain

It is just cleaner this way and makes it easier to add support for
other HW generations with always-on power wells powering a different
set of domains.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>