drm/i915: Introduce for_each_shared_dpll()

No one really cares how we store the shared_dplls. Currently
it happens to be an array, but we could change that to a more
flexible scheme at some point. Hide the implementation details
behind an iterator macro.

The slight downside is the pll variable moving out of the
loop scope, but maybe someday soon we'll start to convert
everything over to having declarations within for-statements...

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/20231003200620.11633-4-ville.syrjala@linux.intel.com