drm/mgag200: Fix VBLANK interrupt handling

Fix support for VBLANK interrupts on G200ER, G200EV and G200SE, which
use a slightly different implementation than the others. The original
commits forgot to update the custom helpers when adding interrupt
handling for VBLANK events.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 89c6ea20 ("drm/mgag200: Add vblank support")
Fixes: d5070c9b ("drm/mgag200: Implement struct drm_crtc_funcs.get_vblank_timestamp")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Acked-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240731071004.519566-1-tzimmermann@suse.de