drm/amdgpu: respect the abmlevel module parameter value if it is set

Currently, if the abmlevel module parameter is set, it is possible for
user space to override the ABM level at some point after boot. However,
that is undesirable because it means that we aren't respecting the
user's wishes with regard to the level that they want to use. So,
prevent user space from changing the ABM level if the module parameter
is set to a non-auto value.

Tested-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>