ion: Add support for dynamically assigned VMIDs

The ION trusted VM flag may map to a VMID that was dynamically
assigned, instead of statically allocated. Thus, use the RM driver
APIs to determine the VMID for the trusted VM.

Change-Id: Ide4dd77a4cedbdf7e32f3c834fef9a0dfc6ba9d5
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>