drm/i915: Extract skl_check_wm_level() and skl_check_nv12_wm_level()

Make the code more typo proof by extracting small helpers that
do the "do we have enough DDB for the WM level?" checks in
a consistent manner.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210305153610.12177-6-ville.syrjala@linux.intel.com


Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>