Revert "drm/amdgpu: don't allow userspace to create a doorbell BO"

This reverts commit 6be2ad4f.

This patch was to block userspace to use doorbell manager UAPI
until usermode queue UAPI gets approved. UQ UAPI got approved in the
following MR:
https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/392



Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>