Xrandr unable to change scale successfully when panning exchange from "2048x2048" to "1024x600"

Submitted by wang,jinjin

Assigned to Chris Wilson @ickle

Link to original bug (#29997)

Description

Created attachment 38404
xorg log

System Environment:
--------------------------
Arch: i386
Platform: Pineview
Libdrm: (master)2.4.21-18-g23287f05cf2443ddf9e028e29beb5bd30979c6cf
Mesa: (master)76d0541e79d4fe2ffcb25b17f9dd540fafc14ba2
Xserver: (master)xorg-server-1.9.0
Xf86_video_intel: (master)2.12.0-76-gb7a8087f
Cairo: (master)f5632d641c44a143386ca3b413e90ed3b1a85f89
Kernel: (for-linus)76be97c1fc945db08aae1f1b746012662d643e97

Bug detailed description:
-------------------------
With the newest commit (for-linus)76be97c1fc945db08aae1f1b746012662d643e97, I found that if we firstly change xrandr's panning from "1024x600" to "2048x2048",then "2048x2048" to "1024x600", the next change of scale will be fail if you move mouse for a little time.But that will not happen, if we change scale before exchanging panning as said above.

Reproduce steps:
----------------
1.xinit&
2.gnome-session&
3.xrandr --output LVDS --panning 2048x2048
4.xrandr --output LVDS --panning 1024x600
5.xrandr --output LVDS --scale 0.5x1
6.moving mouse

Attachment 38404, "xorg log":
Xorg.log