drm/i915/error: Fix object page offset within a region

io_mapping_map_wc() expects the offset to be relative to the iomapping
base address. Currently we just pass in the physical address for the
page which only works if the region.start starts at zero.

Signed-off-by: CQ Tang <cq.tang@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210119133106.66294-2-matthew.auld@intel.com