drm/ttm: Simplify ttm_dma_find_pool() (v2)

Flip the logic of the comparison and remove
the redudant variable for the pool address.

(v2): Remove {} bracing.

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>