drm/i915: Replace wm.max_levels with wm.num_levels and use it everywhere

Replaces wm.max_level with wm.num_levels, since that generally
results in nicer looking code (for-loops can be in standard
form etc.).

Also get rid of the two different wrappers we have for this
(ilk_wm_max_level() and intel_wm_num_levels()). They don't
really do anything for us other than potentially slow things
down if the compiler actually emits the function calls every
time (num_planes*num_wm_levels*higher_level_wm_function_calls
could be a big number). The watermark code already shows up
far too prominently in cpu profiles. Though I must admit that
I didn't look at the generated code this time.

v2: Fix the ilk_wm_merge() off-by-one (Jani)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230209222504.31478-1-ville.syrjala@linux.intel.com