drm/amdgpu: add missing unreserve on error

The amdgpu_bo_unreserve() has to be done on the error path as well.

Fixes: 9e5d2753 ("drm/amdgpu: Move kfd_mem_attach outside reservation")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>