gpu/drm: ingenic: Set max FB height to 4095

While the LCD controller can effectively only support a maximum
resolution of 800x600, the framebuffer's height can be much higher,
since we can change the Y start offset.

v2: No change

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210144142.33143-4-paul@crapouillou.net
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Acked-by: Sam Ravnborg <sam@ravnborg.org>