drm/radeon: add support for read reg query from radeon info ioctl

This allows us to query certain registers from userspace
for profiling and harvest configuration.  E.g., it can
be used by the GALLIUM_HUD for profiling the status of
various gfx blocks.

Tested-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>