drm/amd: add Kconfig dependency for ACP on DRM_AMDGPU

The DRM_AMD_ACP option doesn't have any dependencies and selects
MFD_CORE, which results in MFD_CORE=y.  Since the code is only called
from DRM_AMDGPU, it should depend on it.  Adding the dependency results
in MFD_CORE being selected as a module again if amdgpu is also a module.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>