drm/msm/hdmi: wait a bit for HPD sense to come up

It can take a while for the HPD sense to come up once we've configured
it. If we call HDMI connector's detect() very soon after configuring
HPD registers, drm might think we don't have HDMI connected. Wait a
bit before we let drm use the connector funcs.

Signed-off-by: Archit Taneja <architt@codeaurora.org>