drm/i915: Move the GTFIFODBG to the common mmio dbg framework

Remove the per-mmio checking of the FIFO debug register into the common
conditional mmio debug handling. Based on patch from Chris Wilson.

v2: postpone warn on fifodbg for unclaimed reg debugs

Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>