drm/amdgpu: force read discovery file if set discovery=2

If discovery is set to 2 in module parameters explicitly, the
intention is to use the discovery file in FW rather than the one in
BIOS, usually because the latter is incorrect. This patch to force
read discovery file if set discovery=2.

Signed-off-by: Yifan Zhang <yifan1.zhang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>