drm/rockchip: analogix_dp: Apply devm_clk_get_optional() for &rockchip_dp_device...

drm/rockchip: analogix_dp: Apply devm_clk_get_optional() for &rockchip_dp_device.grfclk

The "grf" clock is optional for Rockchip eDP controller(RK3399 needs
while RK3288 and RK3588 do not).

It can make the code more concise to use devm_clk_get_optional()
instead of devm_clk_get() with extra checks.

In addtion, DRM_DEV_ERROR() is replaced by dev_err_probe().

Signed-off-by: Damon Ding <damon.ding@rock-chips.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250928103734.4007257-1-damon.ding@rock-chips.com