drm/radeon: restore nomodeset operation (v2)

When UMS was deprecated it removed support for nomodeset commandline
we really want this in distro land so we can debug stuff, everyone
should fallback to vesa correctly.

v2: oops -1 isn't used anymore, restore original behaviour
-1 is default, so we can boot with nomodeset on the command line,
then use radeon.modeset=1 to override it for debugging later.

Cc: stable@vger.kernel.org
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>