drm/i915/dpll: replace BUG_ON() with drm_WARN_ON()

Avoid BUG_ON(). Actually check the dpll count and bail out loudly with
drm_WARN_ON() from the loop before overflowing
i915->dpll.shared_dplls[].

v2: Rebase

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220830093411.1511040-2-jani.nikula@intel.com