drm/amd/amdgpu: fix offset into page with amdgpu_iomem debugfs file

The offset inside the page wasn't included in the copy call meaning
the start of the page was being read/written instead.

Reported-by: Jay Cornwall <Jay.Cornwall@amd.com>
Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>