freedreno: UBWC allocator

freedreno: UBWC allocator

UBWC requires space for a metadata or flag buffer
that contains compression data. Each 16x4 tile of image
data corresponds to a byte of compression data.

This buffer needs to be stored before (at a lower address)
the image buffer in order to match up with what the
display driver. This allows the display driver to directly
scan-out at UBWC buffer.