drm/amdgpu: fix runpm documentation

It's not only supported by HG/PX laptops.  It's supported
by all dGPUs which supports BOCO/BACO functionality (runtime
D3).

BOCO - Bus Off, Chip Off.  The entire chip is powered off.
       This is controlled by ACPI.
BACO - Bus Active, Chip Off.  The chip still shows up
       on the PCI bus, but the device itself is powered
       down.

v2: fix missed HG/PX reference

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>