drm/amdgpu: implement si_read_bios_from_rom

This allows us to read the vbios image directly from ROM.
This is already implemented for other asics, but was not
yet available for SI.

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