layers: Unify cb lookup in CmdBeginRenderPass
layers: Unify cb lookup in CmdBeginRenderPass

We already have a GLOBAL_CB_NODE ptr here, and we know it's not null.

Signed-off-by: Chris Forbes <c3279c50043e18e9442c1ae071cafa0e7a4ca967@google.com>

Radamanthe/VulkanSamples,Radamanthe/VulkanSamples,Radamanthe/VulkanSamples,Radamanthe/VulkanSamples,Radamanthe/VulkanSamples,Radamanthe/VulkanSamples