drm/irq: track the irq installed in drm_irq_install in dev->irq

To get rid of the dev->bus->get_irq callback we need to pass in the
desired irq explicitly into drm_irq_install. To avoid having to do the
same for drm_irq_unistall just track it internally. That leaves
drivers with less room to botch things up.

v2: Add the hunk lost in an earlier patch to this one (Thierry).

v3: Fix up the totally fumbled logic in drm_irq_install and use the
local variable consistently. Spotted by both Thierry and Laurent.
Shame on me for failing to properly test the rebase version of this
patch ...

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>