drm/rockchip: vop2: Demote message in mod_supported to drm_dbg_kms

Checking if a modifier is supported by a plane is normal behavior. It is
normal that a plane may not support certain modifiers. Failing the check
doesn't justify an error message in the kernel log and may mislead
users.

Demote the error message to drm_dbg_kms to only print the message if the
respective debug messages are enabled. This is similar to the behavior
in rockchip_drm_vop.c.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20231009103753.830458-1-m.tretter@pengutronix.de