drm/msm/sde: add out of bound check for vsync select

In vsync select, the number of pingpongs is not checked
against number of encoders, which may cause an out of
bound access if an invalid of number of physical encoders
is given. Add a check to validate the number of physical
encoders and pingpongs before configuring vsync select.

Change-Id: Ic84bedfc3e69980126ca60f9dd31b6440dc055ab
Signed-off-by: Alan Kwong <akwong@codeaurora.org>