drm/amdgpu: fix offset calculation in amdgpu_vm_bo_clear_mappings()

commit 5e61b84f9d3ddfba73091f9fbc940caae1c9eb22 upstream.

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

CC: stable@vger.kernel.org
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>