drm/amdgpu/atomfirmware: add parser for gfx_info table

Add support for the gfx_info table on boards that use atomfirmware.

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>