drm/amdgpu: switch to common helper to read bios from rom

create a common helper function for soc15 and onwards
to read bios image from rom

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>