drm/rockchip: vop use is_enabled instead of dpms mode

drm dpms have many power modes: ON,OFF,SUSPEND,STANDBY, etc.
but vop only have enable/disable mode, maybe case such bug:
 --> DRM_DPMS_ON: power on vop
 --> DRM_DPMS_SUSPEND: power off vop
 --> DRM_DPMS_OFF: already power off at SUSPEND, crash
so use a bool val is more suitable.

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