drm/msm/hdmi: enable lpm-mux if it is present

lpm-mux is programmed to enable HDMI connector
on the docking station for S805 chipset based
devices.

Signed-off-by: Beeresh Gopal <gbeeresh@codeaurora.org>
Signed-off-by: Stephane Viau <sviau@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>