meta-agl-bsp: meta-sancloud: weston-ini-conf: Set correct gbm-format

meta-agl-bsp: meta-sancloud: weston-ini-conf: Set correct gbm-format

The default gbm-format needs to be changed on the BeagleBone Enhanced
due to a hardware errata on the AM335x SoC.

For further details see the following section of the TI wiki:
http://processors.wiki.ti.com/index.php/Processor_Linux_SDK_Graphics_and_Display#AM3_Beagle_Bone_Black_Board_Configuration

The TI documentation recommends further changes to powervr.ini and
qt_env.sh, however it was found that only the gbm-format change in
weston.ini is needed.

Change-Id: I968384d0014fef2285d83902de044a5e97274d84
Signed-off-by: Paul Barker <paul@betafive.co.uk>