drm/mediatek: Add gamma property according to hardware capability

If there is no gamma function in the crtc
display path, don't add gamma property
for crtc

Fixes: 2f3f4dda ("drm/mediatek: Add gamma correction.")
Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>