drm/tilcdc: Make tilcdc_crtc_page_flip() work if crtc is not yet on

Make tilcdc_crtc_page_flip() work if crtc is not yet on. The plane
commit sometimes comes before crtc is turned on. The new framebuffer
should be set to scanout also in that case, so that it is there when
crtc is turned on at the end of the commit phase.

Signed-off-by: Jyri Sarha <jsarha@ti.com>