drm/amdgpu: Add module parameter for specifying default ABM level

[Why]
It's non trivial to configure or specify an ABM reduction level for
userspace outside of X. There is also no method to specify the default
ABM value at boot time.

A parameter should be added to configure this.

[How]
Expose a module parameter that can specify the default ABM level to
use for eDP connectors on DC enabled hardware that loads the DMCU
firmware.

The default is still disabled (0), but levels can range from 1-4. Levels
control how much the backlight can be reduced, with being the least
amount of reduction and four being the most reduction.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: David Francis <david.francis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>