drm/tegra: Convert to using __drm_atomic_helper_crtc_reset() for reset.

Convert tegra to using __drm_atomic_helper_crtc_reset(), instead of
writing its own version. Instead of open coding destroy_state(),
call it directly for freeing the old state.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Jonathan Hunter <jonathanh@nvidia.com>
Cc: linux-tegra@vger.kernel.org
[mlankhorst: Keep tegra_crtc_reset at the same place and predeclare
 tegra_crtc_atomic_destroy_state (tagr)]
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301125627.7285-15-maarten.lankhorst@linux.intel.com

drivers/gpu/drm/tegra/dc.c

@@ -26,6 +26,9 @@
 #include <drm/drm_atomic_helper.h>
 #include <drm/drm_plane_helper.h>
 
+static void tegra_crtc_atomic_destroy_state(struct drm_crtc *crtc,
+					    struct drm_crtc_state *state);
+
 static void tegra_dc_stats_reset(struct tegra_dc_stats *stats)
 {
 	stats->frames = 0;
@@ -1155,20 +1158,12 @@ static void tegra_dc_destroy(struct drm_crtc *crtc)
 
 static void tegra_crtc_reset(struct drm_crtc *crtc)
 {
-	struct tegra_dc_state *state;
+	struct tegra_dc_state *state = kzalloc(sizeof(*state), GFP_KERNEL);
 
 	if (crtc->state)
-		__drm_atomic_helper_crtc_destroy_state(crtc->state);
-
-	kfree(crtc->state);
-	crtc->state = NULL;
-
-	state = kzalloc(sizeof(*state), GFP_KERNEL);
-	if (state) {
-		crtc->state = &state->base;
-		crtc->state->crtc = crtc;
-	}
+		tegra_crtc_atomic_destroy_state(crtc, crtc->state);
 
+	__drm_atomic_helper_crtc_reset(crtc, &state->base);
 	drm_crtc_vblank_reset(crtc);
 }