The xf86 mode setting code was mis-using this field to try and store a
pointer to a DisplayModeRec, however, each output has its own copy of every
DisplayModeRec leaving the one in in the RRModeRec devPrivate field pointing
at a random DisplayModeRec.
Instead of attempting to rectify this, eliminating the devPrivate entirely
turned out to be very easy; the DDX code now accepts an arbitrary RRModeRec
structure and set that to the hardware, converting it on the fly to a
DisplayModeRec as needed.
(cherry picked from commit 3506b9376c2b0db09bfff58d64e07af88a6e8195)
9d0c3b52f2