drm/msm/sde: consolidate sde hw irq logic to one spinlock

Consolidate SDE hardware interrupt logic onto a single
spinlock to prevent race conditions between driver and IRQ
context. Also add wmb calls to ensure that IRQ registers have
been written to hardware when adjusting the SDE hardware IRQ
masks and other information.

Change-Id: I4d944d711018101f878aacb469117901384166e9
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>