drm/msm: Add a helper function for in-kernel buffer allocations

Nearly all of the buffer allocations for kernel allocate an buffer object,
virtual address and GPU iova at the same time. Make a helper function to
handle the details.

Change-Id: Ic0dedbad0ecd85d360895cc0d1e418277ba44c62
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>