drm/radeon/kms: viewport height has to be even

Otherwise, no vblank interrupts.
Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=37522



Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>