drm/gma500: Unify crtc helpers

CDV crtc helpers are identical to other chips so use gma_ prefix for the
crtc helper struct and remove the CDV copy.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210201132617.1233-4-patrik.r.jakobsson@gmail.com