drm/i915: Move i915_power_well_regs struct into i915_power_well_ops

Move the i915_power_well_regs struct into i915_power_well_ops. Most of
the power wells use the same ops/regs combination, so this saves some
space and also simplifies the platform power domain->power well
definitions.
Signed-off-by: NImre Deak <imre.deak@intel.com>
Reviewed-by: NJouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220222165137.1004194-5-imre.deak@intel.com