drm/amd/amdgpu/amdgpu_doorbell_mgr: Correct misdocumented param 'doorbell_index'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/amdgpu_doorbell_mgr.c:123: warning: Function parameter or member 'doorbell_index' not described in 'amdgpu_doorbell_index_on_bar'
 drivers/gpu/drm/amd/amdgpu/amdgpu_doorbell_mgr.c:123: warning: Excess function parameter 'db_index' description in 'amdgpu_doorbell_index_on_bar'

Reviewed-by: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>