sna: Select the correct depth for addfb in test_has_dirtyfb()

It's 32bpp, depth 24 (for x8r8g8b8 pixel format), not 32 for everything.
Just to be on the safe side, pick the more common x8r8g8b8 format.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>