drm/radeon: add new callback for info ioctl register accessor

This adds a callback for each asic family to determine what
registers are allowed to be read back via the info ioctl.

The idea here is to allow usermode to query things like GPU status
registers or GPU harvest registers for profiling and determining
the gfx config.

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