video/fbdev/riva: Remove dead code

These are dead code since 3.15. These can be removed forever if no
plan to use it further.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1587407244-32574-1-git-send-email-jrdr.linux@gmail.com