drm/mediatek: add a error return value when clock driver has been prepared

DRM driver get the comp->clk by of_clk_get(), we only
assign NULL to comp->clk when error happened, but do
not return the error number.

Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>