anv: Implement VK_KHR_imageless_framebuffer

anv: Implement VK_KHR_imageless_framebuffer

v2: Pass pointer instead of struct instance (Lionel)

v3: 1) Fix small nits (Jason)
    2) Add way to detect anv_framebuffer don't have attachments (Jason)
    3) Get rid of unncessary pNext chain walk (Jason)
    4) Keep framebuffer instance in anv_cmd_state (Jason)

v4: 1) Dump attachments from cmd_buffer (Jason)

v5: 1) Fix condition check and add assertion (Lionel)

Signed-off-by: Sagar Ghuge <sagar.ghuge@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>