drm/amdgpu: Set ttm caching flags during bo allocation

The ttm caching flags (ttm_cached, ttm_write_combined etc) are
used to determine a buffer object's mapping attributes in both
CPU page table and GPU page table (when that buffer is also
accessed by GPU). Currently the ttm caching flags are set in
function amdgpu_ttm_io_mem_reserve which is called during
DRM_AMDGPU_GEM_MMAP ioctl. This has a problem since the GPU
mapping of the buffer object (ioctl DRM_AMDGPU_GEM_VA) can
happen earlier than the mmap time, thus the GPU page table
update code can't pick up the right ttm caching flags to
decide the right GPU page table attributes.

This patch moves the ttm caching flags setting to function
amdgpu_vram_mgr_new - this function is called during the
first step of a buffer object create (eg, DRM_AMDGPU_GEM_CREATE)
so the later both CPU and GPU mapping function calls will
pick up this flag for CPU/GPU page table set up.

v2: rebase (Alex)

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Suggested-by: Christian Koenig <Christian.Koenig@amd.com>
Reviewed-by: Christian Koenig <Christian.Koenig@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Tested-by: Po Huang <Po.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>