video: omap2: Add support for VSYNC notify via sysfs

 * Sending uevents at 60Hz is very nasty for userspace with many
   listeners. This patch provides an alternate notification mechanism
   via the framebuffer sysfs which we can simply poll on.
 * Requires a userspace change to hwcomposer to support this.

Based on Change-Id Iba180da52ae05b3e702790e27c7ded65d57445c9 from
the msm8660-common CM kernel for Samsungs

[steven@steven676.net: make apply to encore kernel]

Change-Id: I32e1158ee63aab026eed361d39aecbbedfd23dad