drm/i915/display: Fix early deref of 'dsb'

drivers/gpu/drm/i915/display/intel_dsb.c:177 intel_dsb_reg_write() warn: variable dereferenced before check 'dsb' (see line 175)

Fixes: afeda4f ("drm/i915/dsb: Pre allocate and late cleanup of cmd buffer")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Animesh Manna <animesh.manna@intel.com>
Cc: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200524233900.25598-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/display/intel_dsb.c

@@ -169,15 +169,17 @@ void intel_dsb_indexed_reg_write(const struct intel_crtc_state *crtc_state,
 void intel_dsb_reg_write(const struct intel_crtc_state *crtc_state,
 			 i915_reg_t reg, u32 val)
 {
-	struct intel_dsb *dsb = crtc_state->dsb;
 	struct intel_crtc *crtc = to_intel_crtc(crtc_state->uapi.crtc);
 	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
-	u32 *buf = dsb->cmd_buf;
+	struct intel_dsb *dsb;
+	u32 *buf;
 
+	dsb = crtc_state->dsb;
 	if (!dsb) {
 		intel_de_write(dev_priv, reg, val);
 		return;
 	}
+
 	buf = dsb->cmd_buf;
 	if (drm_WARN_ON(&dev_priv->drm, dsb->free_pos >= DSB_BUF_SIZE)) {
 		drm_dbg_kms(&dev_priv->drm, "DSB buffer overflow\n");