drm/msm/a6xx: use msm_gem for GMU memory objects

This gives more fine-grained control over how memory is allocated over the
DMA api. In particular, it allows using an address range or pinning to
a fixed address.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>