drm/sun4i: rgb: Validate the clock rate

Our pixel clock cannot reach a high enough rate for some rather high while
common resolutions (like 1080p60).

Make sure we filter the resolutions we cannot reach in our mode_valid
function.

Fixes: 29e57fab ("drm: sun4i: Add RGB output")
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>