drm/i915: Extract skl_wm_latency()

Extract the skl+ wm latency determination into a small helper
so that everyone has the same idea what the latency should be.

This introduces a slight functional change in that
skl_cursor_allocation() will now start to account for the
extra 4 usec that the kbk/cfl/cml IPC w/a adds.

v2: Rebase

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230301162449.26672-2-ville.syrjala@linux.intel.com