drm/radeon: fix UBSAN warning in kv_dpm.c

commit a498df54 upstream.

Adds bounds check for sumo_vid_mapping_entry.

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>