drm/i915: Fix FDI M/N setting according with correct color depth

FDI M/N calculation hasn't taken the current pipe color depth into account,
but always set as 24bpp. This one checks current pipe color depth setting,
and change FDI M/N calculation a little to use bits_per_pixel first, then
convert to bytes_per_pixel later.

This fixes display corrupt issue on Arrandle LVDS with 1600x900 panel
in 18bpp dual-channel mode.

Cc: Stable Team <stable@kernel.org>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>