drm/amdgpu: cleanup IB pool handling a bit

Fix the coding style, move and rename the definitions to
better match what they are supposed to be doing.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>