layers: Replace is_depth() in image layer with util function
layers: Replace is_depth() in image layer with util function

vk_layer_utils has function vk_format_is_depth_or_stencil() that is
exactly the same as local is_depth() function so kill local function
and use utility function.

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