msm: kgsl: Initialize the return value for adreno_regulator_enable

In the case where the regulator is not already enabled an undefined
value will be returned. Initialize the return value for
adreno_regulator_enable.

Change-Id: If481b631bc5e785c156413a4f74aeacc23ab601a
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>