drm/etnaviv: request pages from DMA32 zone when needed

Some Vivante GPUs are found in systems that have interconnects restricted
to 32 address bits, but may have system memory mapped above the 4GB mark.
As this region isn't accessible to the GPU via DMA any GPU memory allocated
in the upper part needs to go through SWIOTLB bounce buffering. This kills
performance if it happens too often, as well as overrunning the available
bounce buffer space, as the GPU buffer may stay mapped for a long time.

Avoid bounce buffering by checking the addressing restrictions. If the
GPU is unable to access memory above the 4GB mark, request our SHM buffers
to be located in the DMA32 zone.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>