drm/rockchip: vop: set vop enabled after enable iommu

there is a Bug that:
  vop_enable()->drm_vblank_on, drm_vblank_on may call vop
enable vblank. if it happen, vblank enable would failed,
then cause irq status error. because is_enabled value is set
after drm_vblank_on.

after enable vop clocks and iommu regs, we can sure that
R/W vop regs and do vop plane flip is safe, so place
is_enabled = true after enable iommu is suitable.

Signed-off-by: Mark Yao <mark.yao@rock-chips.com>