drm/msm: avoid memory allocation when sgt is imported

When sgt 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>
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>