tests: use drmtest_skip to check for rings

tests: use drmtest_skip to check for rings

To simplify things add a set of gem_check_<ring> functions which take
care of this. Since I've opted for static inlines drmtest.h grew a few
more header includes which was a neat opportunity to dump a few redundant
#defines.

This kills all the skipped_all hand-rolled logic we have.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>