drm/rockchip: fix clk enable disable mismatch in vop_crtc_mode_set

The function disables the dclk at the beginning, so don't simply return
when an error happens, but instead enable the clock again, so that
enable and disable calls are balanced.

ret_clk is introduced to hold the clk_enable result and not mangle the
original error code.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>