drm/i915: add intel_using_power_well

It returns true if we've requested to turn the power well on and it's
really on. It also returns true for all the previous gens.

For now there's just one caller, but I'm going to add more.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>