drm/amdgpu: fix kmap error handling for bo creations

kmap happens after bo pin, so unpin is required on error

Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>