drm/amdgpu: expand runpm parameter

Allow the user to specify -2 as auto enabled with displays.

By default we don't enter runtime suspend when there are
displays attached because it does not work well in some
desktop environments due to the driver sending hotplug
events on resume in case any new displays were attached
while the GPU was powered down.  Some users still want
this functionality though, so this lets you enable it.

Link: drm/amd#2428


Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>