rockchip: video: vop: Fix rk_display_init() return error

It's caused by the difference of clk_set_rate function implement between
rk3288 andd rk3399.

clk_set_rate() of rk3288 return 0 in normal condition.
clk_set_rate() of rk3399 return input parameter in normal condition.

So check clk_set_rate's return value by IS_ERR_VALUE.

Signed-off-by: Eric Gao <eric.gao@rock-chips.com>