drm/amdgpu:cleanup firmware.fw_buf alloc/free

use bo_create/free_kernel instead of manually doing it

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