drm/i915: Use enum plane_id in SKL wm code

Nuke skl_wm_plane_id() and just use the new intel_plane->id.

v2: Convert skl_write_plane_wm() as well
v3: Convert skl_pipe_wm_get_hw_state() correctly
v4: Rebase due to changes in the wm code
    Drop the cursor FIXME from the total data rate calc (Paulo)
    Use the "[PLANE:%d:%s]" format in debug print (Paulo)

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Lyude <cpaul@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479830524-7882-4-git-send-email-ville.syrjala@linux.intel.comReviewed-by: Lyude <lyude@redhat.com>