drm/i915: add HAS_POWER_WELL

We're starting to add many IS_HASWELL checks for the power well code,
so add a HAS_POWER_WELL macro to properly document that we're checking
for hardware that has the power down well.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
[danvet: Resolve conflicts since some converted code was added by
not-yet merged patches.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>