The fragment_extra structure contains additional fields extending the
MRT framebuffer descriptor, snuck in between the main framebuffer
descriptor and the render targets. Its fields include those related to
transaction elimination and depth/stencil buffers. This patch identifies
the flags field (previously just "unk" with some magic values) as well
as identifying some (but not all) flags set by the driver.

The process of identifying flags brought a bug to light where
transaction elimination (checksumming) could not be enabled unless AFBC
was in-use. This issue is now resolved.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>