drm/tegra/dc: Use drm_mode_get_hv_timing() to populate plane clip rectangle

Use drm_mode_get_hv_timing() to fill out the plane clip rectangle.

No functional changes as the code already uses crtc_state->mode
to populate the clip, which is also what drm_mode_get_hv_timing()
uses.

Once everyone agrees on this we can move the clip handling into
drm_atomic_helper_check_plane_state().

v2: Rebase due to tegra_plane_state_add() relocating to plane.c

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: linux-tegra@vger.kernel.org
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123190502.28449-13-ville.syrjala@linux.intel.com

drivers/gpu/drm/tegra/plane.c

@@ -82,7 +82,7 @@ int tegra_plane_state_add(struct tegra_plane *plane,
 {
 	struct drm_crtc_state *crtc_state;
 	struct tegra_dc_state *tegra;
-	struct drm_rect clip;
+	struct drm_rect clip = {};
 	int err;
 
 	/* Propagate errors from allocation or locking failures. */
@@ -90,10 +90,9 @@ int tegra_plane_state_add(struct tegra_plane *plane,
 	if (IS_ERR(crtc_state))
 		return PTR_ERR(crtc_state);
 
-	clip.x1 = 0;
-	clip.y1 = 0;
-	clip.x2 = crtc_state->mode.hdisplay;
-	clip.y2 = crtc_state->mode.vdisplay;
+	if (crtc_state->enable)
+		drm_mode_get_hv_timing(&crtc_state->mode,
+				       &clip.x2, &clip.y2);
 
 	/* Check plane state for visibility and calculate clipping bounds */
 	err = drm_atomic_helper_check_plane_state(state, crtc_state, &clip,