bcm2708_fb : Implement blanking support using the mailbox property interface

This implementation uses the userspace request style of an array of unsigned
32bit ints rather the vc_msg/vc_msg_tag which is more confusing to work with.

vcio.h : Added some extra enums to the vcio.h to improve readability
vcio.h : Renamed DEVICE_FILE_NAME to something more appropriate. users of the
vcio api will be unaffected  by this change as the device node is created manually
in userspace