David Francis authored Jan 03, 2019 and Harry Wentland committed Jan 08, 2019

Adaptive Backlight Management (ABM) is a power-saving
feature on AMD ASICs that reduces backlight while increasing
pixel contrast and luminance.  This test confirms that
ABM is present and enabled, and that backlight performance
is sane.  It uses AMD-specific debugfs entries to
read the backlight PWM values.

It has 5 subtests:

dpms_cycle
Sets brightness to half, then confirms that value is restored
after dpms off and then on.

backlight_monotonic_basic
Sets brightness to ten different values, confirming that
higher brightness values are brighter.

backlight_monotonic_abm
Same as backlight_monotonic_basic, but with abm enabled.

abm_enabled
Sets abm to its four intensity levels, confirming that
abm reduces the backlight, and the reduction is greater
for higher abm level.

abm_gradual
Sets abm to off and then maximum intensity, confirming
that brightness decreases continually over the first
second and eventually reaches the target value.
This test takes 30s to run.

v2: make sure that dpms is cycled on the eDP display

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>