drm: fix documentation for drm_crtc_set_mode()

commit 4c9287c6009b37754c42e0ba73a4cc79de92d8f8 upstream.

x and y parameters are offsets, not width/height

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Weng Meiling <wengmeiling.weng@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>