drm/ingenic: Fix driver not probing when IPU port is missing

Even if support for the IPU was compiled in, we may run on a device
(e.g. the Qi LB60) where the IPU is not available, or simply with an old
devicetree without the IPU node. In that case the ingenic-drm refused to
probe.

Fix the driver so that it will probe even if the IPU node is not present
in devicetree (but then IPU support is disabled of course).

v2: Take a different approach

Fixes: fc1acf31 ("drm/ingenic: Add support for the IPU")
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200827114404.36748-2-paul@crapouillou.net