hwc: Support HDMI as primary display

This patch disables mirroring/docking logic when HDMI display
is used as primary instead of LCD panel.

Based on patch from Mark Tyler.

Rebased from p-ics-mr1:
9dd8ccd Support HDMI as primary display

Change-Id: Idd239a562bfc486e128b268fa31ced53412bba3a
Signed-off-by: Mykola Ostrovskyy <mykola@ti.com>
Signed-off-by: Muralidhar Dixit <murali.dixit@ti.com>
Signed-off-by: Tony Lofthouse <a0741364@ti.com>