drm/fsl-dcu: implement irq_preinstall/uninstall callbacks

Make use of the irq_preinstall/uninstall callback to clear and
mask all interrupts. Use write 1 to clear as documented by the
data sheet (writing a 0 seems to have cleared interrupt status
too). Remove fsl_dcu_drm_irq_init and call drm_irq_install
directly from fsl_dcu_load makes error handling a bit simpler.
Do not set irq_enabled since drm_irq_install is taking care of
it.

Signed-off-by: Stefan Agner <stefan@agner.ch>