xf86drm: introduce drm_device_has_rdev() helper

Currently we match the opened drmDevice fd with each drmDevice we
process.

Move that after all the devices are processed and folded, via the
drm_device_has_rdev(). This makes the code easier to follow and allows
us to unify the massive process loop across drmGetDevice2 and
drmGetDevices2. That in itself is coming with a later commit.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>