msm: kgsl: move memdesc_sg_dma from the header

memdesc_sg_dma is too big to be an inline function so move it into the
code and rename it to match the namespace.

Change-Id: Ic0dedbad859b4ef3e1d168aabbf27970bf7e1288
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>