drivers/amdgpu: Remove redundant NULL check before kfree()

Found-by: Coccinelle
Signed-off-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-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>