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

No functional changes as the code already uses crtc_state->mode
to populate the clip, which is also what drm_mode_get_hv_timing()
uses.

Once everyone agrees on this we can move the clip handling into
drm_atomic_helper_check_plane_state().

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: nouveau@lists.freedesktop.org
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123190502.28449-11-ville.syrjala@linux.intel.comReviewed-by: NThierry Reding <treding@nvidia.com>