drm/i915: Clean up verify_wm_state()

drm/i915: Clean up verify_wm_state()

Get rid of the nonsense cursor special case in verify_wm_state()
by just iterating through all the planes. And let's use the
canonical [PLANE:..] style in the debug prints while at it.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210226153204.1270-8-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>