drm/sde: remove warn on from sde encoder

The sde encoder wait for commit done is called from kms
driver. It can be called during first commit without
enabling the encoder to check if any commits are pending.
That should not be treated as warning.

Change-Id: I6072b3ea1bdea6b93c1ddc04b8d38dc9fa790de3
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>