drm/amdgpu/display - only update eDP's backlight level when necessary

[Why]
The original logic is to update eDP's backlight level
on every amdgpu dm atomic commit, which causes excessive
DMUB write. As a result, when playing game or moving window
around, DMUB timeout and system lagging are observed.

[How]
We only need to update eDP's backlight level when current level
doesn't match requested level.

Signed-off-by: Zhan Liu <zhan.liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>