drm/i915: s/plane_res_b/blocks/ etc.

Rename a bunch of the skl+ watermark struct members to
have sensible names. Avoids me having to think what
plane_res_b/etc. means.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210305153610.12177-7-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>