drm/amd: Drop abm_level property

This vendor specific property has never been used by userspace
software and conflicts with the panel_power_savings sysfs file.
That is a compositor and user could fight over the same data.

Fixes: 63d0b872

 ("drm/amd/display: add panel_power_savings sysfs entry to eDP connectors")
Suggested-by: Harry Wentland <Harry.Wentland@amd.com>
Cc: Hamza Mahfooz <Hamza.Mahfooz@amd.com>
Cc: "Sun peng Li (Leo)" <Sunpeng.Li@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>