drm: xilinx: crtc: Remove the priv_plane field

The primary plane is referred by base_crtc->primary. There's no need
for struct xilinx_drm_crtc to have another pointer to the primary plane.
Use the base_crtc->primary consistently.

Signed-off-by: Hyun Kwon <hyun.kwon@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>