drm/rockchip: vop: Correct enabled clocks during setup

When doing the initial setup both the hclk and the aclk need to be
enabled otherwise the board will simply hang. This only occurs when
building the vop driver as a module, when its built-in the initial setup
happens to run before the clock framework shuts of unused clocks
(including the aclk).

While there also switch to doing prepare and enable in one step rather
then separate steps to reduce the amount of code required.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-by: Mark Yao <mark.yao@rock-chips.com>
Tested-by: Yakir Yang <ykk@rock-chips.com>
Tested-by: Romain Perier <romain.perier@gmail.com>