Yes it has DRM
Yes it has V4L2
It has support for mainline, although there's a LTS 4.14 kernel maintained by manufacturer which has fixes and additional drivers that are not in mainline (e.g. Mali GPU drivers :D)
I've been running leia on DRM/GBM for quite some time (I have a branch of krypton with DRM patches too). With a modified ffmpeg, you can even run drmprime decoder and renderer:
(mali drivers don't support dmabuf_import atm, so only drmprime direct-to-plane can be used. ffmpeg patches are required to convert in hardware and with dmabuf support the NV12 output from decoder to RGB that is accepted by the exynos DRM. also kernel patches are required to enable overlay plane, since by default there's only the primary plane. ideally, with new drivers with dmabuf_import support, we can use drmprime-egl renderer with no patching required).