drm/omapdrm: Track IRQ state in local device state

Replace usage of struct drm_device.irq_enabled with the driver's
own state field struct omap_drm_device.irq_enabled. The field in
the DRM device structure is considered legacy and should not be
used by KMS drivers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210625082222.3845-16-tzimmermann@suse.de