drm/ttm: Fix address passed to dma_mapping_error() in ttm_pool_map()

check_unmap() is producing a warning about a missing map error check.
The return value from dma_map_page() should be checked for an error, not
the caller-provided dma_addr.

Fixes: d099fc8f

 ("drm/ttm: new TT backend allocation pool v3")
Signed-off-by: Jeremy Cline <jcline@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/413432/
Signed-off-by: Christian König <christian.koenig@amd.com>