ion: change data type of source_vm* variable to int

The source_vm* variable is used to hold enum values of type
vmid{}, which has negative values (VMID_INVAL = -1). This variable
is declared as u32 in few functions. Change it to type int.

Change-Id: I4cc546e79a20a843d6bd096ab696913ab40dde6d
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>