drm/i915: clean up plane bpp confusion

- There is no 16bpc linear color format in our hw. gen4+ has a 16 bpc
  float layout, but we don't really support it.
- 10bpc is a gen4+ feature, fix up the support for it.
- Update_plane should never see a wrong fb bpp value, BUG in the
  corresponding cases.

v2: Rebase on top of Ville's plane pixel layout changes.

v3: Actually drop the old gen4 check for 10bpc planes, spotted
by Ville Syrjälä.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>