drm/i915: Split intel_read_wm_latency() into per-platform versions

No reaon to have this humongous if ladder in intel_read_wm_latency().
Just split it into nicer per-platforms functions.

Also do the s/dev_priv/i915/ while touching all of this code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220908191646.20239-2-ville.syrjala@linux.intel.com


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