msm: gsi: Change user_data to support OOO completions

Add a valid flag to user data elements to specify if it is in use.
Add extra escape buffers equal to the MAX_VEID
since there can only be that many TREs on hold.

Change-Id: I898a8ceda921f5a7da2a83c7d7a3565441bfa16e
Signed-off-by: Michael Adisumarta <madisuma@codeaurora.org>