DVDCodecs: Amlogic: Handle conditions in which amcodec should be opened during Open()

DVDCodecs: Amlogic: Use mpeg2 video codec for mpeg2's

* AM Video Codec cannot properly handle the still-frames within DVDs

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>

DVDFactoryCodec: Remove throttling of mpeg2 to libmpeg2 for amlogic

* Thanks to @kszaq, am-mpeg2 can handle all mpeg2's however it can't handle the still frames (which is caught above it).
* Testing .VOB isn't setting hint.stills to true so will need to address this in separate patch

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>

DVDFactoryCodec: Further conditionalize HAS_LIBAMCODEC opening

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>

DVDFactoryCodec: Amlogic: Force rmvb to ffmpeg

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>

DVD: Amcodec: Handle conditions in which amcodec should be using during open

* Instead of before

Signed-off-by: Brandon McAnsh <brandonm@matricom.net>
Merge pull request #4539 from Matricom/amcodec

DVDCodecs: Condense and update HAS_LIBAMCODEC