drm: bridge: dw-hdmi: Don't forward HPD events to DRM core before attach

Hotplug events should only be forwarded to the DRM core by the interrupt
handler when the bridge has been attached, otherwise the DRM device
pointer will be NULL, resulting in a crash.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170117082910.27023-7-laurent.pinchart+renesas@ideasonboard.com