disp: msm: sde: avoid disabling dsc encoder in PP block

DSC encoder block disables the right DSC block on
pingpong for right-only or left-only partial update
frame trigger after a full frame update. This would
cause issue with posted-start feature as the register
is not double buffered. Avoid disabling in PingPong
block as the DSC encoder being used can be controlled
by the DSC common_mode setting.

Change-Id: I20791b925c1a6f8694673e2f8d43e5283305c131
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>