drm/i915/dsb: Fix DSB command buffer size checks

free_pos is in dwords, DSB_BUF_SIZE in bytes. Directly
comparing the two is nonsense. Fix it up, and make sure
we also account for the 8byte alignment requirement for
each instruction, and also assume that each instruction
normally eats two dwords.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221216003810.13338-5-ville.syrjala@linux.intel.comReviewed-by: Animesh Manna <animesh.manna@intel.com>

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

@@ -97,7 +97,7 @@ void intel_dsb_indexed_reg_write(struct intel_dsb *dsb,
 	u32 *buf = dsb->cmd_buf;
 	u32 reg_val;
 
-	if (drm_WARN_ON(&dev_priv->drm, dsb->free_pos >= DSB_BUF_SIZE)) {
+	if (drm_WARN_ON(&dev_priv->drm, ALIGN(dsb->free_pos, 2) > DSB_BUF_SIZE / 4 - 2)) {
 		drm_dbg_kms(&dev_priv->drm, "DSB buffer overflow\n");
 		return;
 	}
@@ -167,7 +167,7 @@ void intel_dsb_reg_write(struct intel_dsb *dsb,
 	struct drm_i915_private *dev_priv = to_i915(crtc->base.dev);
 	u32 *buf = dsb->cmd_buf;
 
-	if (drm_WARN_ON(&dev_priv->drm, dsb->free_pos >= DSB_BUF_SIZE)) {
+	if (drm_WARN_ON(&dev_priv->drm, ALIGN(dsb->free_pos, 2) > DSB_BUF_SIZE / 4 - 2)) {
 		drm_dbg_kms(&dev_priv->drm, "DSB buffer overflow\n");
 		return;
 	}