tests: host1x: gr2d-clear: Add on-screen display support
tests: host1x: gr2d-clear: Add on-screen display support

If the host1x backend supports on-screen display of framebuffers, prefer
that over writing out a PNG file.

Signed-off-by: Thierry Reding <819d67e53b6c9983dea118cea592f5214247b065@gilfi.de>

digetx/grate,digetx/grate,grate-driver/grate,grate-driver/grate