msm: pm: null check before accessing the pointer

Move the l2 cache low power mode read code after the null
check of the rs_limits pointer.

CRs-Fixed: 587781
Change-Id: I51441f92830b1c458a212b1414c15a86fef7f8f7
Signed-off-by: Anil kumar mamidala <amami@codeaurora.org>