drm/amd/display: send pipe set command to dmcu when backlight is set

[Why]
Previously, a change removed code that would send a pipe set command
to dmcu each time the backlight was set, as it was thought to be
superfluous. However, it is possible for the backlight to be set
before a valid pipe has been set, which causes DMCU to hang after a
DPMS restore on some systems.

[How]
Send a pipe set command to DMCU prior to setting the backlight.

Signed-off-by: Josip Pavic <Josip.Pavic@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>