drm/radeon/kms: detect sideport memory on IGP chips

This detects if the sideport memory is enabled and
if it is VRAM is evicted on suspend/resume.

This should fix s/r issues on some IGPs.

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