drm/radeon: Avoid NULL pointer dereference from atom_index_iio() allocation failure

Smatch anlysis:

drivers/gpu/drm/radeon/atom.c:1242 atom_index_iio() error: potential null
 dereference 'ctx->iio'.  (kzalloc returns null)

Also cleaned up some checks before calls to kfree(). kfree(NULL) is OK.

Cc: David Airlie <airlied@linux.ie>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Michel Dänzer" <michel.daenzer@amd.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>