drm/amdgpu: set CPU mapping of vram as cached for A+A mode

New A+A HW supports cached vram mapped to cpu.

Signed-off-by: Eric Huang <jinhuieric.huang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>