drm/amd/amdgpu: add pm lock to debugfs mmio entry

Adds support for PM locks around access to registers that might
have race conditions on PG transistions.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>