cpuidle: lpm-levels: Fix NULL pointer exception at lpm_get_latency

In cases where client driver would call the API lpm_get_latency before the
lpm_root_node is initialized, would result in a NULL pointer exception. Fix
the same by moving the node check before.

CRs-Fixed: 973895
Change-Id: Ib2c7257bb7289f68d2bcb621ad311ceefe88a5f3
Signed-off-by: Taniya Das <tdas@codeaurora.org>