drm/radeon: rename alt_domain to allowed_domains

And also domain to prefered_domains. That matches better
what those values represent.

Signed-off-by: Christian König <christian.koenig@amd.com>
Cc: Marek Olšák <maraeo@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>