msm: lmh_lite: Use mutex when accessing DPM register

Use the LMH scm mutex when writing to DPM registers.
This is to avoid any access from LMH driver when APM
switch is initiated.

CRs-Fixed: 900507
Change-Id: I8683b505de9f3b8dc11f8076b4ba3889bdef45fb
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>