drm/radeon/ci add comment to document intentionally unreachable code

 commit d967be9b ("drm/radeon/ci: disable needless sclk changes")
 introduces an unreachable if(C != C) conditional code section
 flagged by coccinelle script bad_conditional.cocci:

 Add a comment to make it clear that this is intentional.

 Fixes: d967be9b ("drm/radeon/ci: disable needless sclk changes")

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>