drm/amdgpu: Move to gtt before cpu accesses dma buf.

drm/amdgpu: Move to gtt before cpu accesses dma buf.

To improve cpu read performance. This is implemented for APUs currently.

v2: Adapt to change https://lists.freedesktop.org/archives/amd-gfx/2017-October/015174.html
v3: Adapt to change "forward begin_cpu_access callback to drivers"
v4: Instead of v3, reuse drm_gem dmabuf_ops here. Also some minor fixes as suggested.
v5: only set dma_buf ops when it is valid (Samuel)

Signed-off-by: Samuel Li <Samuel.Li@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>