drm/radeon/kms: fix typo in radeon_compute_pll_gain

Looks like this got copied from the ddx wrong.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>