drm/radeon: fix error handling in radeon_driver_open_kms

The return value was never initialized so the cleanup code executed when
it isn't even necessary.

Just add proper error handling.

Fixes: ab50cb9d ("drm/radeon/radeon_kms: Fix a NULL pointer dereference in radeon_driver_open_kms()")
Signed-off-by: Christian König <christian.koenig@amd.com>
Tested-by: Jan Stancek <jstancek@redhat.com>
Tested-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>