drm/msm: Initial add eDP support in msm drm driver (v5)

This change adds a new eDP connector in msm drm driver. With this
change, eDP panel can work with msm platform under drm framework.

v1: Initial change

v2: Address Rob's comments
    Use generated header file for register definitions
    Change to devm_* APIs

v3: Address Thierry's comments and rebase on top of atomic changes
    Remove edp_bridge_mode_fixup
    Remove backlight control code and rely on pwm-backlight
    Remove continuous splash screen support for now
    Change to gpiod_* APIs

v4: Fix kbuild test issue

Signed-off-by: Hai Li <hali@codeaurora.org>
[robclark: v5: rebase on drm_bridge changes in drm-next]
Signed-off-by: Rob Clark <robdclark@gmail.com>