drm/i915: Use a common function for computing the fb height alignment

If we need to change the fb height constraints, it sounds like a good
idea to have to do it in one place only.

v2: v2: Rebase on top of Ander's "Make intel_crtc->config a pointer"

Reviewed-By: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v1)
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>