amdsmu: Read and dump idlemask

Read and dump the SMU's idlemask value in dev.amdsmu.0.idlemask
sysctl.

The idlemask is used internally by AMD and its value is not documented
anywhere. This patch exposes it mainly to help AMD diagnose issues with
S0i3 entry on FreeBSD.

Reviewed by: cem, emaste, kib, mckusick (mentor)
Approved by: emaste, kib, mckusick (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51487