drm/ingenic: Silence uninitialized-variable warning

Silence compiler warning about used but uninitialized 'ipu_state'
variable. In practice, the variable would never be used when
uninitialized, but the compiler cannot know that 'priv->ipu_plane' will
always be NULL if CONFIG_INGENIC_IPU is disabled.

Silence the warning by initializing the value to NULL.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200719093834.14084-1-paul@crapouillou.net