modesetting: reverse prime support (v1.1)

This adds support for reverse prime to the modesetting driver.

Reverse prime is where we have two GPUs in the display chain,
but the second GPU can't scanout from the shared pixmap, so needs
an extra copy to the on screen pixmap.

This allows modesetting to support this scenario while still
supporting the USB offload one.

v1.1:
fix comment + ret = bits (Eric)

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>