qseecom: set 64bit dma mask

Set 64bit dma mask for qseecom to perform dma on 64bit address
properly. Also reset local dmabuf pointers after unmapping dmabuf
to avoid double free.

Change-Id: I4ead045721c290fdb7742dbea3d5de614d703420
Signed-off-by: Zhen Kong <zkong@codeaurora.org>