drm/amdgpu: split amdgpu_device_access_vram() into two small parts

split amdgpu_device_access_vram()
1. amdgpu_device_mm_access(): using MM_INDEX/MM_DATA to access vram
2. amdgpu_device_aper_access(): using vram aperature to access vram (option)

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>