drm/gpuvm: Helper to get range of unmap from a remap op.

Determining the start and range of the unmap stage of a remap op is a
common piece of code currently implemented by multiple drivers. Add a
helper for this.

Changes since v7:
- Renamed helper to drm_gpuva_op_remap_to_unmap_range()
- Improved documentation

Changes since v6:
- Remove use of __always_inline

Signed-off-by: Donald Robson <donald.robson@imgtec.com>
Signed-off-by: Sarah Walker <sarah.walker@imgtec.com>
Reviewed-by: Danilo Krummrich <dakr@redhat.com>