msm: vidc: Halt axi bus before clock disable

Before turning off venus clock, host should make sure
that there are no pending AXI transactions. This is achieved
by halting AXI.

CRs-Fixed: 578007
Change-Id: Ib4a71d1097f2f62e910e00c7be5d32e006d4c62b
Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>