drm: amd: Resolve Sphinx unexpected indentation warning

Resolves Sphinx unexpected indentation warning when compiling
documentation (e.g. `make htmldocs`). Replaces tabs with spaces and adds
a literal block to keep vertical formatting of the
example power state list.

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com> (v2)
Acked-by: Randy Dunlap <rdunlap@infradead.org> (v2)
Signed-off-by: Hunter Chasens <hunter.chasens18@ncf.edu>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>