drm/i915: Use intel_PLL_is_valid() in vlv_find_best_dpll()

Everyone else uses intel_PLL_is_valid(), so make VLV use it as well.

We don't have any special p and m limits on VLV, so skip those tests,
and we also need to skip the m1<=m2 test line PNV.

Reorganize the function a bit to move the n check alongside the rest of
the test for the non-derived dividers, and check the derived values
afterwards.

Note that this changes vlv_find_best_dpll() in two ways:
- The .vco comparison is now >max instead of >=max, and since we round
  down when calculating that stuff, we may now allow frequencies slightly
  above the max as we do on other platforms. The previous method
  disallowed exactly max and anything above it.
- We now check the .dot frequency against the data rate limits, which we
  didn't do before.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>