drm/ttm: Fix coding style in ttm_dma_pool_alloc_new_pages()

Add missing {} braces.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>