drm/gma500: Remove dead code

These are dead code since 3.7. If there is no plan to use them further,
these can be removed forever.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1587406337-32317-1-git-send-email-jrdr.linux@gmail.com