drm/amdgpu: Enable scatter gather display support

Enables sg display if vram size <= THRESHOLD(256M); otherwise
still use vram as display buffer.
This patch fixed some potention issues introduced by change
"allow framebuffer in GART memory as well" due to CZ/ST hardware
limitation.

v2: Change default setting to auto.
v3: Move some logic from amdgpu_display_framebuffer_domains()
    to pin function, suggested by Christian.
v4: Split into several patches.
v5: Drop module parameter for now.

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