layers: Add validation for VkDescriptorBufferInfo struct

layers: Add validation for VkDescriptorBufferInfo struct

For a descriptor write update to a buffer type, verify that the contents of
VkDescriptorBufferInfo struct are valid according to valid usage guidelines
of the spec.

Spun off the usage bit validation into its own function ValidateBufferUsage().
Previous ValidateBufferUpdate() function now calls that function as well as
performing all of the checks for the buffer, range, and offset members of
VkDescriptorBufferInfo struct.