drm/msm/sde: change print message from error to debug

Print message was previously changed to SDE_ERR during
development, but should be SDE_DEBUG. This change
corrects that to avoid printing an unjustified error message.

Change-Id: I15c453dda167367ec98a049a8c1e02f82d6f627d
Signed-off-by: Samantha Tran <samtran@codeaurora.org>