drm/gma500: Remove unused declarations and other cruft

Most of these are old leftovers from one of the driver merges. This is
all dead code.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220317092555.17882-1-patrik.r.jakobsson@gmail.com