drm/amdgpu: fix NULL pointer dereference for Renoir

Commit c1cf79ca ("drm/amdgpu: use IP discovery table for renoir")
introduced a NULL pointer dereference when booting with
amdgpu.discovery=0, because it removed the call of vega10_reg_base_init()
for that case.

Fix this by calling that funcion if amdgpu_discovery == 0 in addition to
the case that amdgpu_discovery_reg_base_init() failed.

Fixes: c1cf79ca ("drm/amdgpu: use IP discovery table for renoir")
Signed-off-by: Dirk Gouders <dirk@gouders.net>
Cc: Hawking Zhang <Hawking.Zhang@amd.com>
Cc: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>