If the power well is disabled, we should not try to read its
registers, otherwise we'll get "unclaimed register" messages.

V2: Don't check whether the power well is enabled or not, just check
whether we asked it to be enabled or not: if we asked to disable the
power well, don't use the registers on it, even if it's still enabled.

V3: Fix bug that breaks all non-Haswell machines.
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>