drm/rockchip: dw_hdmi: introduce the pclk for grf

For RK3399's GRF module, if we want to operate the graphic related grf
registers, we need to enable the pclk_vio_grf which supply power for VIO
GRF IOs, so it's better to introduce an optional grf clock in driver.

Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Mark Yao <mark.yao@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>