msm: sde: Fix SDE rotator context list locking

During rotator IRQ handling, it is possible that the context list
getting updated with open and close handling within the IRQ timeframe,
so it is necessary to add the spinlock to protect the context list.

CRs-Fixed: 1087423
Change-Id: I20a04c766e55c75db15662e9a13e20396b35f30e
Signed-off-by: Benjamin Chan <bkchan@codeaurora.org>