drm/sun4i: hdmi: Consolidate atomic_check and mode_valid

atomic_check and mode_valid do not check for the same things which can
lead to surprising result if the userspace commits a mode that didn't go
through mode_valid. Let's merge the two implementations into a function
called by both.

Acked-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240222-kms-hdmi-connector-state-v7-35-8f4af575fce2@kernel.org