gma500: handle poulsbo cursor restriction

Poulsbo needs a physical address in the cursor base register. We allocate a
stolen memory buffer and copy the cursor image provided by userspace into it.
When/If we get our own userspace driver we can map this stolen memory directly.
The patch also adds a mark in chip ops so we can identify devices that has this
requirement.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>