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>
Change-Id: I9e883c0700696a0884adbeaeba14bde3270e98fa