xf86drm: Handle unrecognized subsystems safely in drmGetDevice[s]()

Both drmGetDevice() and drmGetDevices() currently print a warning when
they encounter an unknown (non-PCI) subsystem type for a device node,
but they still proceed to assume that the drmDevicePtr was initialized
and try to add it to the local device array.  Add a 'continue' to the
error case handling to bypass the rest of the processing for devices we
can't handle.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>