anv: Refactor get_buffer_format_properties()
Make it a stand-alone function. Pre-patch, for some formats the function
returned incorrect VkFormatFeatureFlags which were cleaned up by the
caller.
This prepares for a cleaner implementation of
VK_EXT_image_drm_format_modifier.
Reviewed-by:
Jason Ekstrand <jason@jlekstrand.net>