drm/dp: via debugfs entry node perform power transition

The change will make a debugfs entry into
/sys/kernel/debug/drm_dp/dpms_mode.
echo 1: normal mode
echo 2: power down mode
echo 5: power down mode with AUX block fully powered.

Change-Id: I7b04e4884ad0fdb531f94b5d5cabf6578c0c39e2
Signed-off-by: Shubham Talekar <quic_stalekar@quicinc.com>