[rfc] drm/radeon/kms: pm debugging check for vbl.

This patch adds a check on avivo chips to see if we are in the VBL
region for the active crtcs when we trigger the engine change.

I appear to have glitches locally on pm transistion (not sure all
fixes are in yet) and this at least seems to be correct here,
maybe others can test on systems with no glitches.