drm/armada: clean up armada_drm_crtc_page_flip()

drm_mode_page_flip_ioctl() already takes care of checking the
framebuffer format, and also assigns primary->fb after a successful
call to this handler.  These are both redundant, and can be removed.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>