drm/mgag200: Add workaround for HW that does not support 'startadd'

commit 1591fadf857cdbaf2baa55e421af99a61354713c upstream.

There's at least one system that does not interpret the value of
the device's 'startadd' field correctly, which leads to incorrectly
displayed scanout buffers. Always placing the active scanout buffer
at offset 0 works around the problem.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: John Donnelly <john.p.donnelly@oracle.com>
Tested-by: John Donnelly <john.p.donnelly@oracle.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: 81da87f6 ("drm: Replace drm_gem_vram_push_to_system() with kunmap + unpin")
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: "Y.C. Chen" <yc_chen@aspeedtech.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "José Roberto de Souza" <jose.souza@intel.com>
Cc: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v5.3+
Link: https://gitlab.freedesktop.org/drm/misc/issues/7
Link: https://patchwork.freedesktop.org/patch/msgid/20191126101529.20356-4-tzimmermann@suse.de


[drop debugfs_init callback - gregkh]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>