drm/gma500: Constify static structs

The only usage of these is to assign their address to the fbops field in
the fb_info struct, which is a const pointer. Make them const to allow
the compiler to put them in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200830211741.17326-1-rikard.falkeborn@gmail.com