drm/i915: fix VLV limits

Magic updates.

v2: use 64 bit types and math (Ville)

v3: Trim out all the m/n/p calculation changes since they are still
under discussion. Instead squash in a fixup for hdmi limits which
slipped into a different patch.

Signed-off-by: Pallavi G <pallavi.g@intel.com>
Signed-off-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Signed-off-by: Yogesh M <yogesh.mohan.marimuthu@intel.com>
Signed-off-by: Gajanan Bhat <gajanan.bhat@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org> (v2)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>