drm/amdgpu: fix offset calculation in amdgpu_vm_bo_clear_mappings()

Offset calculation wasn't correct as start addresses are in pfn
not in bytes.

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>