drm/i915: use id from intel_shared_dpll.info

Replace all users of pll->id to use pll->info->id. In functions using
this more than once it was preferred to add an id variable to make the
code easier to read.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180320220637.21480-6-lucas.demarchi@intel.com