drm/msm/sde: don't gate frame done on ppsplit slave encoders

Fix to avoid adding pingpong split slave encoders to the virtual
encoder frame busy mask since they do not handle irqs, they will
prevent the frame done from firing.

CRs-Fixed: 2005394
Change-Id: I2f9604a6bd78405a2fdbe1127ae2519ca92c2665
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>