bcm2709_fb: refine appropriate behaviors to unsupported fb ioctls

since fbturbo introduces and use FBUNSUPPORTED ioctl on copyarea
operations for unsupported dummy ioctl which is expected to return
failure. in such scenario, bcm2709 always prompts error log.

in order not to bother users in kernel log, we change the dev_err to
dev_dbg and return a ENOTTY other than EINVAL to let userspace handles
the return value.

Signed-off-by: wuyuehang <yuehan9.wu@gmail.com>
Reviewed-by: popcornmix <popcornmix@gmail.com>
Reviewed-by: Phil Elwell <pelwell@users.noreply.github.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>