msm: adsprpc: decrease length of IOVA mask for SDSP devices

Two bits are sufficient to append all valid stream IDs for SDSP
fastrpc context banks for sm8150 as a software workaround. This
will eliminate the possibility of invalid IOVA generation.

Change-Id: I66a23d291a40331c0693a8dd31013b8a1ea545c7
Acked-by: Thyagarajan Venkatanarayanan <venkatan@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>