msm: mdss: Avoid used pipe list corruption

Since a pipe can be freed through an overlay release call or during
kickoff, the !list_empty check is needed before deleting the
used_list head to avoid corruption by attempting to delete the
same head twice.

Change-Id: I80ecce7bddbdf443e3570d408652a35a99758b07
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>