drm/msm: check hpd_off flag only for pluggable displays

This change will wrap the 'hpd_off' flag with pluggable displays.
In non-pluggable display case 'hpd_off' flag prevents disabling
of HPD clocks and hence display subsystem doesn't got into suspend
state.

Change-Id: I28ade8fb60a60a15f1d23009de63bbf9f10c7c77
Signed-off-by: Rahul Sharma <rahsha@codeaurora.org>
Signed-off-by: Suprith Malligere Shankaregowda <supgow@codeaurora.org>