drm/tilcdc: Change tilcdc_crtc_page_flip() to tilcdc_crtc_update_fb()

Change tilcdc_crtc_page_flip() to tilcdc_crtc_update_fb(). The
function is not used as a page_flip() callback anymore so it is only
confusing to call it that. The function should only be used by dummy
primary plane commit() callback.

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