HDP non surface should cover the whole VRAM but we were misscomputing
the size and we endup in some case not covering the VRAM at all (if
VRAM size were > 1G). Covering more than the VRAM size shouldn't be
an issue.

Fix : https://bugs.freedesktop.org/show_bug.cgi?id=28016

[airlied: add evergreen fix]
Signed-off-by: NJerome Glisse <jglisse@redhat.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>