drm/i915: ValleyView mode setting limits and PLL functions

Add some VLV limit structures and update the PLL code.

v2: resolve conflicts, Vijay to re-post with PLL valid checks and fixed limits
v3: re-add dpio write function
v4: squash in Vijay's fixes for the PLL limits and clean up the m/n finder

Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Signed-off-by: Vijay Purushothaman <vijay.a.purushothaman@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>