drm/tilcdc: Precalculate total frametime in tilcdc_crtc_set_mode()

commit ce99f7206c9105851d97202ed08c058af6f11ac4 upstream.

We need the total frame refresh time to check if we are too close to
vertical sync when updating the two framebuffer DMA registers and risk
a collision. This new method is more accurate that the previous that
based on mode's vrefresh value, which itself is inaccurate or may not
even be initialized.

Reported-by: Kevin Hao <kexin.hao@windriver.com>
Fixes: 11abbc9f ("drm/tilcdc: Set framebuffer DMA address to HW only if CRTC is enabled")
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>