drm/amd: Do not make DRM_AMD_ACP default to y

By default, not only this driver is enabled on all platforms, but also
generic PM Domains and Multi-Function Devices.

Drop the "default y" to fix this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>