drm/etnaviv: move linear window on MC1.0 parts if necessary

On i.MX6SX the physical memory is placed above the 2GB mark, so the GPU
linear window has to be moved for the GPU to work at all. This doesn't
mix with the FAST_CLEAR feature, as the TS unit doesn't take the linear
window offset into account and will corrupt memory when used with a
non-zero offset.

Move the linear window if it's necessary for the GPU to work, but avoid
announcing FAST_CLEAR support to userspace in this case.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Marek Vasut <marex@denx.de>