drm: adv7511: Fix crash in IRQ handler when no encoder is associated

The ADV7511 is probed before its slave encoder init function associates
it with an encoder. This creates a time window during which hot plug
detection interrupts can occur with an encoder, resulting in a crash in
the IRQ handler.

Fix this by ignoring hot plug detection IRQs when no encoder is
associated yet.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>