drm/mgag200: Set/clear <syncrst> field in display enable/disable helpers

Modifying the <syncrst> field in mgag200_{enable,disable}_display()
makes the code more readable. Also clear the <asyncrst> field to enable
the display. The other bits in SEQ0 are unused, so no functional changes
are made.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200707082411.6583-6-tzimmermann@suse.de