drm/i915/bxt: make bxt_calc_pll_link() similar to skl

Rename state to pll_state and use it as the argument to
bxt_calc_pll_link(), similar to how it's done in the skl variant.

The WARN_ON(!crtc_state->shared_dpll) is not very useful, so remove it
as well.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190322223751.22089-3-lucas.demarchi@intel.com


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>