msm: mdss: add cmd_off_mtx for synchronizing vsync handler and off sequence

At present the ctl->offlock mutex is used for synchronization
between ESD thread and OFF sequence. Using the same mutex
for synchronization between add_vsync_handler() and OFF
sequence was causing unnecessary synchronization between
ESD thread and add_vsyc_handler(). To improve performance
a new mutex, cmd_off_mtx is used for the second case.

CRs-Fixed: 1051864
Change-Id: I7d0b0690672869937d82df68ea9f571023fc88d1
Signed-off-by: Shalini Krishnamoorthi <shakri@codeaurora.org>