Use drm_mode_get_hv_timing() to fill out the plane clip rectangle.

Since armada isn't atomic we'll use crtc->enabled and crtc->mode instead
of the stuff under crtc_state.

Once everyone agrees on how the clip rectangle gets set up we can
move the code into drm_atomic_helper_check_plane_state().

Cc: Dave Airlie <airlied@redhat.com>
Cc: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180123170857.13818-4-ville.syrjala@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Dave Airlie <airlied@redhat.com>