msm: lpm_levels: Fix logic for choosing cpu mode

When "qcom,allow-synched-level" flag is not defined, the cpu low power
modes that need to be synchronized with the cluster mode are not chosen.
This could result in sub optimal power savings. Fix by disallowing
system low power modes and allowing CPU low power modes.

Change-Id: I3d37054e785cdd3c3e6ded252e351d24a5ff9130
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>