Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/arm/display/komeda/komeda_plane.c:
 In function komeda_plane_atomic_duplicate_state:
drivers/gpu/drm/arm/display/komeda/komeda_plane.c:161:35:
 warning: variable old set but not used [-Wunused-but-set-variable

It is not used since commit 990dee3a ("drm/komeda:
Computing image enhancer internally")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190722055627.38008-1-yuehaibing@huawei.com