media: mediatek: vcodec: Enable HEVC main still picture decode

Mediatek devices that support HEVC also support the main still picture
profile, but today, the main still picture profile is excluded.

This removes the skip mask for HEVC, and enables the main still
picture profile decoding.

Signed-off-by: Nathan Hebert <nhebert@chromium.org>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>