drm/radeon: use a single doorbell for cik kms compute

A single doorbell page is plenty for cik kms compute.
Use a single page and manage doorbell allocation by
individual doorbells rather than pages.  Identify
doorbells by their index rather than byte offset.

Signed-off-by: Andrew Lewycky <Andrew.Lewycky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>