drm/i915: use power domain api to check vga power state

This way we can reuse the check on other platforms too. Also factor out
a version of the function that doesn't check if the power is on, we'll
need to call this from within the power domain framework.

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>