msm: kgsl: Use proper power level indexes in slumber

Before notifying slumber via OOB we set the GX and MX power level
indexes at the specified control registers. This patch updates the GX
and MX votes to the right values.

Change-Id: I4ba8d4d489bb362cf64c808c72599fe40124c5c3
Signed-off-by: Sharat Masetty <smasetty@codeaurora.org>