drm/amd/pm: Add missing mutex for pp_get_power_profile_mode

Prevent possible issues from set and get being called simultaneously.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>