drm/radeon: Prevent multiple debug error lines on suspend

Fix to avoid multiple debug error lines printed on every suspend by Radeon driver's debugfs.

radeon_debugfs_init() calls debugfs_create_file() for every ring.

This results in printing multiple error lines to the screen and dmesg similar to this:

[   92.378726] debugfs: File 'radeon_ring_gfx' in directory '0000:00:01.0' already present!
[   92.378732] debugfs: File 'radeon_ring_cp1' in directory '0000:00:01.0' already present!
[   92.378734] debugfs: File 'radeon_ring_cp2' in directory '0000:00:01.0' already present!
[   92.378737] debugfs: File 'radeon_ring_dma1' in directory '0000:00:01.0' already present!
[   92.378739] debugfs: File 'radeon_ring_dma2' in directory '0000:00:01.0' already present!
[   92.380775] debugfs: File 'radeon_ring_uvd' in directory '0000:00:01.0' already present!
[   92.406620] debugfs: File 'radeon_ring_vce1' in directory '0000:00:01.0' already present!
[   92.406624] debugfs: File 'radeon_ring_vce2' in directory '0000:00:01.0' already present!

Patch v1: The fix was to run lookup() for the file before trying to (re)create that debug file.
Patch v2: Call the radeon_debugfs_init() only once when radeon ring is initialized (as suggested
by Christian K. - thanks)

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Woody Suwalski <terraluna977@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>