drm/amdgpu: Make VRAM discovery read optional

When overridden with module param, directly read discovery info
from discovery binary instead of reading from VRAM.

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