drm/rockchip: dw_hdmi: Simplify clock handling

Make use of devm_clk_get_optional_enabled() to replace devm_clk_get()
and clk_prepare_enable() for ref_clk and drop the now unnecessary calls
to clk_disable_unprepare().

Additionally, use devm_clk_get_optional() helper for grf_clk to replace
the open coding call to devm_clk_get() followed by the -ENOENT test.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240813-dw-hdmi-rockchip-cleanup-v1-2-b3e73b5f4fd6@collabora.com