drm/amdgpu: add an option to override IP discovery table from a file

If you set amdgpu.discovery=2 you can force the the driver to
fetch the IP discovery table from a file rather than from the
table shipped on the device.  This is useful for debugging and
for device bring up and emulation when the tables may be in flux.

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