drm/amdgpu: simplify return value in amdgpu_userq_get_doorbell_index

drm/amdgpu: simplify return value in amdgpu_userq_get_doorbell_index

amdgpu_userq_get_doorbell_index returns a uint64 type index
as well as a int type failure values. Simplifying this and
using a int type return value and getting the index in input pointer
of type uint64 type.

Also since it's used at once place making it static would be better.

Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>