drm: fix drm_can_sleep() comment

Reversed logic when writing the original comment, now fixed.

Fixes: e9eafcb5 ("drm: move drm_can_sleep() to drm_util.h")
Reported-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190120171217.12508-1-sam@ravnborg.org