drm/msm/sde: fix cmd mode vblank irq control

Fix previous merge conflict that disabled the pingpong done irq
on vblank off instead of the pingpong read pointer as intended.

Change-Id: I707437b54f2878d0da7b0d3b5618fdafa0ff38d5
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>