drm/i915: Avoid clearing the base drm_crtc_state

To prevent having to preserve the drm_crtc_state as we clear the
intel_crtc_state, only memset our extended state.

Fixes:
drivers/gpu/drm/i915/intel_display.c: In function ‘clear_intel_crtc_state’:
drivers/gpu/drm/i915/intel_display.c:11301:1: error: the frame size of 1056 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

v2: Add a comment and BUILD_BUG_ON to explain the memset()

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170303154644.6709-1-chris@chris-wilson.co.uk
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -11272,7 +11272,6 @@ clear_intel_crtc_state(struct intel_crtc_state *crtc_state)
 {
 	struct drm_i915_private *dev_priv =
 		to_i915(crtc_state->base.crtc->dev);
-	struct drm_crtc_state tmp_state;
 	struct intel_crtc_scaler_state scaler_state;
 	struct intel_dpll_hw_state dpll_hw_state;
 	struct intel_shared_dpll *shared_dpll;
@@ -11284,7 +11283,6 @@ clear_intel_crtc_state(struct intel_crtc_state *crtc_state)
 	 * fixed, so that the crtc_state can be safely duplicated. For now,
 	 * only fields that are know to not cause problems are preserved. */
 
-	tmp_state = crtc_state->base;
 	scaler_state = crtc_state->scaler_state;
 	shared_dpll = crtc_state->shared_dpll;
 	dpll_hw_state = crtc_state->dpll_hw_state;
@@ -11292,9 +11290,11 @@ clear_intel_crtc_state(struct intel_crtc_state *crtc_state)
 	if (IS_VALLEYVIEW(dev_priv) || IS_CHERRYVIEW(dev_priv))
 		wm_state = crtc_state->wm;
 
-	memset(crtc_state, 0, sizeof *crtc_state);
+	/* Keep base drm_crtc_state intact, only clear our extended struct */
+	BUILD_BUG_ON(offsetof(struct intel_crtc_state, base));
+	memset(&crtc_state->base + 1, 0,
+	       sizeof(*crtc_state) - sizeof(crtc_state->base));
 
-	crtc_state->base = tmp_state;
 	crtc_state->scaler_state = scaler_state;
 	crtc_state->shared_dpll = shared_dpll;
 	crtc_state->dpll_hw_state = dpll_hw_state;