msm: npu: Correct mapped npu mem size

npu mem size should be retrieved from struct dma_buf structure
directly instead of from the first entry of sg_table.

Change-Id: I3ee5555b51781a0d40540a3d8d3fc7e3d3ddef3a
Signed-off-by: Jilai Wang <jilaiw@codeaurora.org>