drm/radeon/kms: add pll quirk for toshiba laptop panel

The panel on the Toshiba A300-1BU laptop does not like the
PLL dividers selected by the avivo pll algo, but works fine
using the older method.  Add a quirk to handle it.

Should fix fdo bug 26358.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>