drm: hdlcd: Cleanup the atomic plane operations

Harden the plane_check() code to drop attempts at scaling because
that is not supported. Make hdlcd_plane_atomic_update() set the pitch
and line length registers that correctly reflect the plane's values.
And make hdlcd_crtc_mode_set_nofb() a helper function for
hdlcd_crtc_enable() rather than an exposed hook.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>