VRR BTR patches revision 2

[Mario Kleiner]

Updated series. The debug patch is dropped, a r-b by Nicholas is tacked
onto patch 1/3 and patch 3/3 has the locking fix that Nicholas proposed.
In terms of testing 3/3 didn't change anything for the better or worse,
observed behaviour on retested DCN-1 and DCE-8 is the same.
Patch 2/3 is identical.

For reference i made a little git repo with a cleaned up version
of my test script VRRTest.m and some pdf's with some plots from my
testing with a slightly earlier version of that script:

https://github.com/kleinerm/VRRTestPlots

Easy to use on a Debian/Ubuntu based system as you can get GNU/Octave
+ psychtoolbox-3 from the distro repo. Explanations in the Readm

The following plots illustrate how patch 2/3 can sometimes help to
make transition to BTR a bit smoother min VRR was 25 Hz or 40 msecs
in this test case on DCE8 and 30 Hz or 33 msecs on DCE11.

Without patch 2/3:

https://github.com/kleinerm/VRRTestPlots/blob/master/VRR_DCE8_2upstepping_VUPDATEbtr+removehysteresispatch.pdf
https://github.com/kleinerm/VRRTestPlots/blob/master/VRR_DCE11_2upstepping_VUPDATEbtr+removehysteresispatch.pdf

With patch 2/3:

https://github.com/kleinerm/VRRTestPlots/blob/master/VRR_DCE11_2upstepping_VUPDATEbtr.pdf
https://github.com/kleinerm/VRRTestPlots/blob/master/VRR_DCE8_2upstepping_VUPDATEbtr.pdf

-mario