drm/radeon: add query to fetch the max engine clock (v2)

This is needed for reporting the max GPU engine clock
in OpenCL.  This just reports the max possible engine
clock, it does not take into account current conditions
that may limit that clock.

v2: fix query number for merge with 3.13

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>