drm/amdgpu: Fix create_dmamap_sg_bo kdoc warnings

Fix the following gcc with W=1:

drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c:292: warning: Cannot understand  * @create_dmamap_sg_bo: Creates a amdgpu_bo object to reflect information

Cc: Felix Kuehling <felix.kuehling@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>