disp: msm: dp: add backlight for edp

Register backlight and cooling devices for eDP interface
and reuse connector backlight update functions. Parse and
use backlight properties from devicetree.

Change-Id: I5d96a8d588d8f7266aa5bffad310718942e7dfe6
Signed-off-by: Venkata Prahlad Valluru <quic_vvalluru@quicinc.com>