drm: Move IRQ related fields to proper section

The .irq and .irq_enabled fields are part of the VBLANK interrupt
handling infrastructure, so move them to the appropriate section within
the structure.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>