OMAPDSS: DISPC: Fix calc_scaling_44xx() bugs for writeback pipeline

dispc_ovl_calc_scaling_44xx() doesn't work correctly for writeback. There are
two issues with it:

- the function tries to calculate pixel clock for the input plane using
  dispc_plane_pclk_rate(), calling this with writeback as input plane results in
  a BUG(), this function shouldn't be called for writeback at all. Fix this by
  calculating pixel clock only when we are not in mem to mem mode.

- the maximum input_width is the product of the downscale ratio supported and
  the and the given output_width. This was calculated incorrectly by dividing
  output_width with maxdownscale. Fix this.

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