disp: msm: sde: use default scale_cfg during plane error

Commit C1 --> plane A with scalar config, encountered an error
	  --> plane A is marked as solidfill.

Commit C2 --> plane A with scale ROI's but no config as it was
	      provided in the earlier commit.

Fix handles the above scenario via allowing fallback onto
default cfg until the next scalar config is provided by usermode.

Plane error can occur too late in the atomic update, hence
atomic duplicate of scalar_cfg might not help in some cases.

Change-Id: I6a75668c285e1dde532ed03f82ddb9be95117977
Signed-off-by: Kalyan Thota <kalyant@codeaurora.org>