drm: xilinx: plane: Remove an unused arg of xilinx_drm_plane_mode_set()

In xilinx_drm_plane_mode_set(), struct drm_crtc is never used, so
the argument is removed from the function prototype.

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