drm/msm/sde: take spin lock around encoder vblank registration

Take sde phyiscal encoder spin lock around the vblank irq
control logic to protect against race conditions between idle
power collapse and the crtc both trying to control vblank irq
registrations.

Change-Id: Ie67e0ae921ad1123585ef9b5e31079a320648273
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>