sde-drm: address memory leak on composer

With the DP MST hotplug use case, a memory leak was found on the
composer. This change will free up the memory used by a temporary
variable on sde-drm side.

Change-Id: I546b335920b0c2d8e1518a3540a47d94ea8721c9
CRs-Fixed: 2715028
3 files changed