drm/mediatek: Fix can't get component for external display plane.

The original logic is ok for primary display, but will not find out
component for external display.

For example, plane->index is 6 for external display, but there are only
2 layer nr in external display, and this condition will never happen:
if (plane->index < (count + mtk_ddp_comp_layer_nr(comp)))

Fix this by using the offset of the plane to mtk_crtc->planes as index,
instead of plane->index.

Fixes: d6b53f68

 ("drm/mediatek: Add helper to get component for a plane")
Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
Signed-off-by: CK Hu <ck.hu@mediatek.com>