drm/amd/display: restore code for plane with no modifiers

When this file was split in commit 5d945cbc ("drm/amd/display:
Create a file dedicated to planes") this chunk seemed to get dropped.

Linus noticed on this rx580 and I've reproduced on FIJI which makes
sense as these are pre-modifier GPUs.

With this applied, I get gdm back.

Fixes: 5d945cbc ("drm/amd/display: Create a file dedicated to planes")
Signed-off-by: Dave Airlie <airlied@redhat.com>
Reported-and-tested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>