Implement detect() by checking the hot plug detect status.

The implementation is not very good, as it always turns on the HDMI
output to get the detection working. HDMI driver needs improvements so
that we could enable only core parts of it.

Cc: Mythri P K <mythripk@ti.com>
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>