drm/amdgpu: define and add extra dword for jpeg ring

Define extra dword for jpeg ring. Jpeg ring will allocate extra dword to store
the patch commands for fixing the known issue.

v2: dropping extra_dw for rings other than jpeg.

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