disp: msm: sde: reduce log level for invalid encoder master

Reduce the log level from error to debug for invalid
master encoder in encoder restore path during idle
power-collapse. This would avoid unnecessary logging
when concurrent writeback gets enabled after a
power-collapse event, the restore happens based on
encoder_mask attached in the crtc before the actual
enable call for the cwb encoder.

Change-Id: I54cd7de62f1a9030f68a963c8ed27ff098302e04
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>