OMAPDSS: DISPC: Fix scaling constraints for OMAP4

The calculation of required DISPC_FCLK for downscaling is done by multplying the
pixel clock with an integer factor. This isn't true for OMAP4 where the required
clock is calculated using the exact ratio of downscaling done.

Fix this calculation for OMAP4. Also, do a minor clean up of calc_fclk().

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