drm: mediatek: use crtc helper drm_crtc_from_index()

Use drm_crtc_from_index() to find drm_crtc for given index, so that we
do not need to maintain a pointer array in struct mtk_drm_private.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1483961145-18453-4-git-send-email-shawnguo@kernel.org