drm/i915: Move adjust_wm_latency() out from {mtl,skl}_read_wm_latency()

drm/i915: Move adjust_wm_latency() out from {mtl,skl}_read_wm_latency()

{mtl,skl}_read_wm_latency() are doing way too many things for
my liking. Move the adjustment stuff out into the caller.
This also gives us one place where we specify the 'read_latency'
for all the platforms, instead of two places.

Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250919193000.17665-6-ville.syrjala@linux.intel.com