drm/ingenic: Validate mode in a .mode_valid callback

Validate modes in the drm_crtc_helper_funcs.mode_valid() callback, which
is designed for this purpose, instead of doing it in
drm_crtc_helper_funcs.atomic_check().

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200728151641.26124-3-paul@crapouillou.net