drm/amdgpu: Add flag to make VBIOS read optional

Certain SOCs may not need much data from VBIOS. Some data like VBIOS
version used will be missed but it doesn't affect functionality. Add a
flag to make VBIOS image optional.

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