msm: ADSPRPC: Unmap buffer when all references are released

Remove hypervisor mapping only when all references to the map
are released and fix the buffer size set for CMA heap.

Change-Id: I852f8f541d0f6cd8d2353e7bf091c24e400c9a9e
Signed-off-by: Sathish Ambley <sathishambley@codeaurora.org>