drm/amd/display: fix default brightness

[Why]
To avoid flickering during boot default brightness level set by BIOS
should be maintained for as much of the boot as feasible.
commit 2fe87f54 ("drm/amd/display: Set default brightness according
to ACPI") attempted to set the right levels for AC vs DC, but brightness
still got reset to maximum level in initialization code for
setup_backlight_device().

[How]
Remove the hardcoded initialization in setup_backlight_device() and
instead program brightness value to match BIOS (AC or DC).  This avoids a
brightness flicker from kernel changing the value.  Userspace may however
still change it during boot.

Fixes: 2fe87f54 ("drm/amd/display: Set default brightness according to ACPI")
Acked-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 0747acf3)
Cc: stable@vger.kernel.org