drm/i915/hsw+: Add support for multiple power well regs

Future platforms increase the number of power wells which require
additional control registers. A convenient way to select the correct
register is to use the high bits of the power well ID as index. This
patch only prepares for this, while upcoming platform enabling patches
will add the actual new power well IDs and corresponding power well
control registers.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Rakshmi Bhatia <rakshmi.bhatia@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Reviewed-by: Rakshmi Bhatia <rakshmi.bhatia@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170814151530.24154-2-imre.deak@intel.com