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.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
[dropped msm_fbdev conversion to new helper, since it interferes with
display-handover work, where we want to separate allocation and mapping]
Signed-off-by: Rob Clark <robdclark@gmail.com>