intel/perf: query register descriptions

intel/perf: query register descriptions

This will be useful when we implement queries using a series of MI_SRM
instead of MI_RPC.

Unfortunately on Gen12, the MI_RPC command sources values from the OAR
unit which has a similar series of register as the OAG unit but some
of the configuration of HW doesn't reach OAR so we have to snapshot
OAG manually instead.

v2: Fix comments
    Use const

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6518>