It's unused. Also most of our tests failed to ask for the right type
of drm fd anyway. So it's imo better to just let them fall over when
they don't get master but want it, like they already do today.

This also allows us to garbage-collect the master parameter to
drm_get_card and associated code.

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