panel-raspberrypi-touchscreen: Round up clk rate to fix DSI panel.

Commit 488f9bc8 slightly increased the
reported rate of PLLD, so the clk driver decided that PLLD/3/8 was now
higher than our requested pixel clock rate and rejected it in favor of
PLLD/4/8, which then ran the pixel clock way out of spec.

By bumping the requested clock rate just slightly, we get back to
PLLD/3/8 like we wanted and the panel displays content again.

Signed-off-by: Eric Anholt <eric@anholt.net>