drm/via: copy DRM_WAIT_ON as VIA_WAIT_ON and use it

VIA_WAIT_ON() is a direct copy of DRM_WAIT_ON() from
drm_os_linux.h.
The copy is made so we can avoid the dependency on the legacy header.
A more involved approach had been to introduce wait_event_* but for this
legacy driver the simpler and more safe approach with a copy of the
macro was selected.
Added the relevant header files for the functions used in VIA_WAIT_ON.

v3:
- Updated users of DRM_WAIT_ON => VIA_WAIT_ON (Emil)
- Updated $subject (Emil)

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/20190723200944.17285-3-sam@ravnborg.org