overlay: Drop legacy mmio access

Before the i915_pmu kernel interface was available, we had to rely on
doing some decidedly dodgy mmio access to registers. However, now that
we have a stable interface via perf for grabbing all the details we
need, that and its supporting infrastructure can be discarded.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Antonio Argenziano <antonio.argenziano@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Acked-by: Antonio Argenziano <antonio.argenziano@intel.com>