OMAPDSS: DPI: widen the pck search when using dss fck

OMAPDSS: DPI: widen the pck search when using dss fck

When not using DSI PLL to generate the pixel clock, but DSS FCK, the
possible pixel clock rates are rather limited. DSS FCK is currently used
on OMAP2 and OMAP3.

When using Beagleboard with a monitor that supports high resolutions,
the clock rates do not match (at least for me) for the monitor's pixel
clocks within the current threshold in the code, which is +/- 1MHz.

This patch widens the search up to +/- 15MHz. The search is done in
steps, i.e. it first tries to find a rather exact clock, than a bit less
exact, etc. so this should not change the cases where a clock was
already found.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>