drm/vc4: Demote user-accessible DRM_ERROR paths to DRM_DEBUG.

drm/vc4: Demote user-accessible DRM_ERROR paths to DRM_DEBUG.

Userspace shouldn't be able to spam dmesg by passing bad arguments.
This has particularly become an issues since we started using a bad
argument to set_tiling to detect if set_tiling was supported.

Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 83753117f1de ("drm/vc4: Add get/set tiling ioctls.")
Link: https://patchwork.freedesktop.org/patch/msgid/20170725162733.28007-1-eric@anholt.net
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>