drm/rockchip: Demote normal drm_err to debug

drm/rockchip: Demote normal drm_err to debug

A plane check failing is a normal and expected condition, as userspace
isn't aware of the specific constraints and will try any and every
combination until one succeeds. Push this down to a debug message, so
users can see it if they want to, but make sure we don't spam the log
during normal operation.

Fixes: 604be85547ce4 ("drm/rockchip: Add VOP2 driver")
Signed-off-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20251015110042.41273-2-daniels@collabora.com