drm/i915: Update comments around base bpp

Forgot to do that in

commit d328c9d7
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Apr 10 16:22:37 2015 +0200

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

and it's confusing. Fix it.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>