compositor-drm: Ignore non-KMS devices

compositor-drm: Ignore non-KMS devices

Given that we can have render-only devices, or vgem in a class of its
own, ignore any non-KMS devices in compositor-drm's device selection.
For x86 platforms, this is mostly a non-issue since we look at the udev
boot_vga issue, but other architectures which lack this, and have
multiple KMS devices present, will hit this.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reported-by: Thierry Reding <treding@nvidia.com>
Reported-by: Daniel Vetter <daniel.vetter@intel.com>