drm/via: make via_drv.h self-contained

Added include of header files to make via_drv.h self-contained.

v3:
- Reworded changelog a little - to reflect that more than one
  header files are added

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Kevin Brace <kevinbrace@gmx.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc: Mike Marshall <hubcap@omnibond.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Michel Dänzer <michel@daenzer.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190723200944.17285-4-sam@ravnborg.org