disp: msm: dp: restore brightness on resume for eDP

For eDP connectors, store brightness setting during
bridge disable and restore during bridge enable.

Signed-off-by: Nilesh Laad <quic_nlaad@quicinc.com>
Change-Id: I04834b86110d9bec40d458f609adb8aeae49b9dd
(cherry picked from commit f76dda60)