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

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 <[email protected]>
Reported-by: John Donnelly <[email protected]>
Tested-by: John Donnelly <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Fixes: 81da87f63a1e ("drm: Replace drm_gem_vram_push_to_system() with kunmap + unpin")
Cc: Gerd Hoffmann <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: David Airlie <[email protected]>
Cc: Sam Ravnborg <[email protected]>
Cc: "Y.C. Chen" <[email protected]>
Cc: Neil Armstrong <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: "José Roberto de Souza" <[email protected]>
Cc: Andrzej Pietrasiewicz <[email protected]>
Cc: [email protected]
Cc: <[email protected]> # v5.3+
Link: https://gitlab.freedesktop.org/drm/misc/issues/7
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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