drm/i915/bxt: BUNs related to port PLL

drm/i915/bxt: BUNs related to port PLL

This patch contains changes based on 2 updates to the spec:
Port PLL VCO restriction raised up to 6700.
Port PLL now needs DCO amp override enable for all VCO frequencies.

v2: Sonika's review comment addressed
	- dcoampovr_en_h variable not required
Based on a discussion with Siva, the following changes have been made.
	- replace dco_amp var with #define BXT_DCO_AMPLITUDE
	- set pll10 in a single assignment

v3:
Move DCO amplitude default value to i915_reg.h. Suggested by Siva.

Signed-off-by: Vandana Kannan <vandana.kannan@intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com> [v2]
[danvet: Spell out BUN since not everyone knows what this means.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>