drm/i915: fix pixel color depth setting on eDP

Original DP mode_valid check didn't take pixel color depth into account,
which made one 1600x900 eDP panel's mode check invalid because of overclock,
but actually this 6bpc panel does can work with x1 lane at 2.7G. This one
trys to take bpp value properly both in mode validation and mode setting.

Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>