drm/irq: Document return values more consistently

Some of the functions are documented inconsistently. Add Returns:
sections where missing and use consistent style to describe the return
value.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --cc drivers/gpu/drm/drm_irq.c
Simple merge