drm/radeon/kms: Force HDP_NONSURF to maximum size

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: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>