drm/msm: avoid segmentation of dma_buf mappings

Ensure dma_buf mappings aren't broken in to multiple segments
by setting the devices segment size and boundary.  This will
improve performance and the sg_dma_len() API will now give
the correct buffer size.

Change-Id: Ic3a3c520ea488d367c8df0f1ce7aca5000e9f3e7
Signed-off-by: Steve Cohen <cohens@codeaurora.org>