drm: kirin: Remove out_format from ade_crtc

As part of refactoring the kirin driver to better support
different hardware revisions, this patch removes the out_format
field in the struct ade_crtc, which was only ever set to
LDI_OUT_RGB_888.

Thus this patch removes the field and instead directly uses
LDI_OUT_RGB_888.

Cc: Rongrong Zou <zourongrong@gmail.com>
Cc: Xinliang Liu <z.liuxinliang@hisilicon.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel <dri-devel@lists.freedesktop.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Xinliang Liu <z.liuxinliang@hisilicon.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Xu YiPing <xuyiping@hisilicon.com>
[jstultz: reworded commit message]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190820230626.23253-6-john.stultz@linaro.org