layers: GH873, Fix ETC2 RGBA format class size
layers: GH873, Fix ETC2 RGBA format class size

These formats were incorrectly specified as using an 8-byte storage
class, but each 4x4 block is actually compressed to 128 bits.

Change-Id: I33353cbe3574f0c640b68c8c3c544a286c79ed49

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools