msm: vidc: update index from unsigned to signed number

[1] if target ramsize value is higher than mentioned in
memory_limit_tbl_mbytes table, then index value will
reach zero in msm_comm_get_memory_limit.
[2] currently index is u32, so for nex iteration index value
will become huge positive value instead of -1.
[3] It will lead to OOB access at memory_limits_tbl[i].ddr_size.
and it will go in infinite loop.

Change-Id: I0bfcb98660af65a1e1852f9a67c2f26dcbdac232
Signed-off-by: Govindaraj Rajagopal <grajagop@codeaurora.org>