drm/i915/icl: split pll enable in three steps

Create separate functions to 1) enable power, 2) write pll config, and
3) enable pll. Doing this it makes it easier to share the functions for
the different PLL types by passing the right arguments.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190309035727.25389-3-lucas.demarchi@intel.com