Reland: Force calling PostDisplayConfigurationChange() even if earyling out in UpdateDisplays

Reland: Force calling PostDisplayConfigurationChange() even if earyling out in UpdateDisplays

When changing from software mirroring mode to sinlge display mode, it
is possible there is no need to update |displays_| and we early out
UpdateDisplays(). But we still want to run the PostDisplayConfigurationChange()
cause there are some clients need to act on this, e.g.
TouchTransformerController needs to adjust the TouchTransformer when
switching from dual displays to single display.

BUG=chrome-os-partner:31868
TEST=tested on Big, after existing software mirroring mode, the touch location
     transformation is still correct.

TBR=oshima@chromium.org

Committed: https://crrev.com/2c7fdae75971a3352f0b29a1a0672cc09050738e
Cr-Commit-Position: refs/heads/master@{#294908}

[Yufeng Shen (Slow to review), 2014-09-15 20:26:05]

miletus@chromium.org changed reviewers:
+ oshima@chromium.org

[Yufeng Shen (Slow to review), 2014-09-15 20:26:24]

This is relanding 
https://codereview.chromium.org/559213002/

[Yufeng Shen (Slow to review), 2014-09-15 20:26:29]

The CQ bit was checked by miletus@chromium.org

[commit-bot: I haz the power, 2014-09-15 20:32:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/551073005/1

[commit-bot: I haz the power, 2014-09-15 22:50:23]

Committed patchset #1 (id:1) as 6e459d9005b4c23623239e7ae69069fe65efdb2d

[commit-bot: I haz the power, 2014-09-15 22:52:53]

Patchset 1 (id:??) landed as
https://crrev.com/2c7fdae75971a3352f0b29a1a0672cc09050738e
Cr-Commit-Position: refs/heads/master@{#294908}