drm/i915: Call {vlv,chv}_prepare_pll() from {vlv,chv}_enable_pll()

We always call the vlv/chv prepare_pll() just before enable_pll().
Move the calls into the enable_pll() funcs. We can also
consolidate the DPLL_VCO_ENABLE checks while at it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210715093530.31711-10-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>