panfrost: Reload framebuffer contents if there's no clear

If by flush time the client hasn't submitted a clear, add jobs for
reloading the framebuffer contents as the first draw in the frame.

This is required by programs such as Weston which don't do clears and
rely on the previous contents of the framebuffer being there.

Reloading the whole framebuffer on every frame without regards to what
is needed or what is going to be covered is very inefficient, but future
work will introduce support for damage regions and partial updates so we
know what needs to be actually reloaded.

Fixes quite a few tests in dEQP-EGL.functional.buffer_age.*.

[Alyssa: The context is that tilers do an implicit glClear() on every
frame, whether you asked them to or not. If you want a clear, this is
very efficient. But if you don't, you have to explicitly blit the
backbuffer back into tile memory, accomplished by a dummy texturing
draw. This patch generates that draw via u_blitter, although we could do
a bit better ourselves by eliding the vertex job. This fixes "black
rectangles in Weston/sway" as well as "video not displaying when UI
visible in mpv"]

Signed-off-by: Tomeu Vizoso <[email protected]>
Reviewed-by: Alyssa Rosenzweig <[email protected]>

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