drm/rockchip: vop2: Drop unnecessary if_pixclk_rate computation

The if_pixclk_rate variable is not being used outside of the if-block in
rk3588_calc_cru_cfg(), hence move the superfluous assignment from the
first branch to the inner comment-block.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Tested-by: FUKAUMI Naoki <naoki@radxa.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20250204-vop2-hdmi0-disp-modes-v3-2-d71c6a196e58@collabora.com