msm: kgsl: set regulator mode before disable it on a640/a680

For a640/a680, we need to actually disable CX GDSC instead of
just return a enable state, the GMU oob set will timeout while
doing gmu idle check, therefor to really turn off CX GDSC
requires to set regulator mode first, and there are several
places to disable cx gdsc, move the set mode to
a6xx_cx_regulator_disable_wait.

Change-Id: Idb7d1b75f17426cc5cb6bb8054b2c2c4400f3db2
Signed-off-by: Hui Li <hul@codeaurora.org>