drm/i915: Select starting pipe bpp irrespective or the primary plane

Since universal planes the primary plane might not be around, and it's
kinda silly to restrict the pipe bpp to the primary plane if we might
end up displaying a 10bpc video overlay. And with atomic we might very
well enable a pipe without a primary plane. So just use the platform
max as a starting point and then restrict appropriately.

Of course this is all still a bit moot as long as we artificially
compress everything to max 8bpc because we don't use the hi-bpc gamma
tables.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>