Replace NVIDIA-specific output checking

If the binary driver drives at least one screen, the old method did
not allow for randr-style screen binding (on the other screen).

Change the logic to:
- check if the output an randr output
- if not, check if the "default" randr output is present and the output name
  was "HEAD-n"
  - if so, use Xinerama-style mapping

This keeps the current behaviour of not allowing Xinerama-style mapping if
all outputs are RandR capable.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Tested-by: Aaron Plattner <aplattner@nvidia.com>