drm/amdgpu: Print once if RAS unsupported

MESA polls for errors every 2-3 seconds. Printing with dev_info() causes
the dmesg log to fill up with the same message, e.g,

[18028.206676] amdgpu 0000:0b:00.0: amdgpu: df doesn't config ras function.

Make it dev_dbg_once(), as it isn't something correctible during boot or
thereafter, so printing just once is sufficient. Also sanitize the message.

Cc: Alex Deucher <Alexander.Deucher@amd.com>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: John Clements <john.clements@amd.com>
Cc: Tao Zhou <tao.zhou1@amd.com>
Cc: yipechai <YiPeng.Chai@amd.com>
Fixes: 8b0fb0e9 ("drm/amdgpu: Modify gfx block to fit for the unified ras block data and ops")
Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Alex Deucher <Alexander.Deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>