drm/amdgpu: fix emit frame size and comments for jpeg

Fix vcn jpeg ring emit fence size in dword, and fix the naming in comments.

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Acked-by: Leo Liu <leo.liu at amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>