drm/amd/display: Fixed dim around 1sec when resume from S3 (v2)

root cause:
DMCU try to perform a smoothness brightness change.Incorrect initial
brightness level causes the 1 sec dim.
Change:
Cache brightness level in stream, and clear it when edp backlight on.
If brightness level in stream is 0, set brightness with ramp value is 0.
DMCU will set the brightness without smoothness transition.

v2: squash in null pointer fix (Harry)

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>