disp: msm: dp: add eDP panel notifier support

If panel notifier support is set, get panel
phandle and add panel to drm_panel. Notifier
for this panel will be called on dpms update.

Change-Id: I6346eb09a551a993ec9050797d1ba9383491e612
Signed-off-by: Nilesh Laad <quic_nlaad@quicinc.com>