drm/amdgpu: use amdgpu_device_vram_access in amdgpu_ttm_access_memory v2

Make use of the better performance here as well.

This patch is only compile tested!

v2: fix calculation bug pointed out by Felix

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Jonathan Kim <Jonathan.Kim@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>