drm/msm: avoid memory allocation when sg table is imported

When sg_table is imported, avoid allocating memory. This will
help in reducing alloc/free overhead there by improving
the composition cycle time.

Change-Id: I1514e88f3ef7f7320719445884142f9ded43c49d
Signed-off-by: Kalyan Thota <kalyant@codeaurora.org>