drm: Remove obsolete "This is gross" comment

Remove obsolete comment which was initially added in 2008 to annotate
that idr_find() was used before idr_remove() since idr_remove() didn't
use to return feedback. The comment now is irrelevant with
commit f6cd7dae ("drm: Release driver references to handle before
making it available again").

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170921210424.GA21951@Haneen