drm/amdgpu: add a callback for reading the bios from the rom directly

This is necessary when the vbios image is not directly accessible via
the rom BAR or legacy vga location.

Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>