drm/i915: Stop requiring PLL index == PLL ID

There's no good reason to keep around this PLL index == PLL ID
footgun. Get rid of it.

Both i915->shared_dplls[] and state->shared_dpll[] are indexed
by the same thing now, which is just the index we get at
initialization from dpll_mgr->dpll_info[]. The rest is all about
PLL IDs now.

v2: Add pll->index to mimic drm_crtc & co.
    Remove the comment saying ID should match the index
v3: s/i/pll->index/ in debugfs loop (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231003200620.11633-2-ville.syrjala@linux.intel.com