drm/amdpgu/debugfs: Simplify some exit paths

To avoid code repetition, unify the function exit path when possible. No
functional changes.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>