drm/rockchip: vop2: Drop unused if_dclk_rate variable

Commit 5a028e8f ("drm/rockchip: vop2: Add support for rk3588")
introduced a variable which ended up being unused:

rockchip_drm_vop2.c:1688:23: warning: variable ‘if_dclk_rate’ set but not used [-Wunused-but-set-variable]

This has been initially used as part of a formula to compute the clock
dividers, but eventually it has been replaced by static values.

Drop the variable declaration and move its assignment to the comment
block, to serve as documentation of how the constants have been
generated.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240105174007.98054-1-cristian.ciocaltea@collabora.com